<!-- $Header: /cvsroot/pgsql/doc/src/sgml/func.sgml,v 1.66 2001/08/15 07:10:12 ishii Exp $ -->

<chapter id="functions">
 <title>Functions and Operators</title>

 <indexterm zone="functions">
  <primary>functions</primary>
 </indexterm>

 <indexterm zone="functions">
  <primary>operators</primary>
 </indexterm>

 <para>
  <productname>Postgres</productname> provides a large number of
  functions and operators for the built-in data types.  Users can also
  define their own functions and operators, as described in the
  <citetitle>Programmer's Guide</citetitle>.  The
  <application>psql</application> commands <command>\df</command> and
  <command>\do</command> can be used to show the list of all actually
  available functions and operators, respectively.
 </para>

 <para>
  If you are concerned about portability then take note that most of
  the functions and operators described in this chapter, with the
  exception of the most trivial arithmetic and comparison operators
  and some explicitly marked functions, are not specified by the <acronym>SQL</acronym>
  standard.  However, many other <acronym>RDBMS</acronym> packages provide a lot of the
  same or similar functions, and some of the ones provided in
  <productname>Postgres</productname> have in fact been inspired by
  other implementations.
 </para>


 <sect1 id="functions-logical">
  <title>Logical Operators</title>

  <indexterm zone="functions-logical">
   <primary>operators</primary>
   <secondary>logical</secondary>
  </indexterm>

  <indexterm>
   <primary>Boolean</primary>
   <secondary>operators</secondary>
   <see>operators, logical</see>
  </indexterm>

  <para>
   The usual logical operators are available:

   <indexterm>
    <primary>and</primary>
    <secondary>operator</secondary>
   </indexterm>

   <indexterm>
    <primary>or</primary>
    <secondary>operator</secondary>
   </indexterm>

   <indexterm>
    <primary>not</primary>
    <secondary>operator</secondary>
   </indexterm>

   <simplelist>
    <member>AND</member>
    <member>OR</member>
    <member>NOT</member>
   </simplelist>

   <acronym>SQL</acronym> uses a three-valued Boolean logic where NULL represents
   <quote>unknown</quote>.  Observe the following truth tables:

   <informaltable>
    <tgroup cols="4">
     <thead>
      <row>
       <entry><replaceable>a</replaceable></entry>
       <entry><replaceable>b</replaceable></entry>
       <entry><replaceable>a</replaceable> AND <replaceable>b</replaceable></entry>
       <entry><replaceable>a</replaceable> OR <replaceable>b</replaceable></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>TRUE</entry>
       <entry>TRUE</entry>
       <entry>TRUE</entry>
       <entry>TRUE</entry>
      </row>

      <row>
       <entry>TRUE</entry>
       <entry>FALSE</entry>
       <entry>FALSE</entry>
       <entry>TRUE</entry>
      </row>

      <row>
       <entry>TRUE</entry>
       <entry>NULL</entry>
       <entry>NULL</entry>
       <entry>TRUE</entry>
      </row>

      <row>
       <entry>FALSE</entry>
       <entry>FALSE</entry>
       <entry>FALSE</entry>
       <entry>FALSE</entry>
      </row>

      <row>
       <entry>FALSE</entry>
       <entry>NULL</entry>
       <entry>FALSE</entry>
       <entry>NULL</entry>
      </row>

      <row>
       <entry>NULL</entry>
       <entry>NULL</entry>
       <entry>NULL</entry>
       <entry>NULL</entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>

   <informaltable>
    <tgroup cols="2">
     <thead>
      <row>
       <entry><replaceable>a</replaceable></entry>
       <entry>NOT <replaceable>a</replaceable></entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>TRUE</entry>
       <entry>FALSE</entry>
      </row>

      <row>
       <entry>FALSE</entry>
       <entry>TRUE</entry>
      </row>

      <row>
       <entry>NULL</entry>
       <entry>NULL</entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </sect1>

 <sect1 id="functions-comparison">
  <title>Comparison Operators</title>

  <indexterm zone="functions-comparison">
   <primary>comparison</primary>
   <secondary>operators</secondary>
  </indexterm>

  <table>
   <title>Comparison Operators</TITLE>
   <tgroup cols="2">
    <thead>
     <row>
      <entry>Operator</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry> <literal>&lt;</literal> </entry>
      <entry>less than</entry>
     </row>

     <row>
      <entry> <literal>&gt;</literal> </entry>
      <entry>greater than</entry>
     </row>

     <row>
      <entry> <literal>&lt;=</literal> </entry>
      <entry>less than or equal to</entry>
     </row>

     <row>
      <entry> <literal>&gt;=</literal> </entry>
      <entry>greater than or equal to</entry>
     </row>

     <row>
      <entry> <literal>=</literal> </entry>
      <entry>equal</entry>
     </row>

     <row>
      <entry> <literal>&lt;&gt;</literal> or <literal>!=</literal> </entry>
      <entry>not equal</entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <note>
   <para>
    The <literal>!=</literal> operator is converted to
    <literal>&lt;&gt;</literal> in the parser stage.  It is not
    possible to implement <literal>!=</literal> and
    <literal>&lt;&gt;</literal> operators that do different things.
   </para>
  </note>

  <para>
   Comparison operators are available for all data types where this
   makes sense.  All comparison operators are binary operators that
   return values of type <type>boolean</type>; expressions like
   <literal>1 &lt; 2 &lt; 3</literal> are not valid (because there is
   no <literal>&lt;</literal> operator to compare a Boolean value with
   <literal>3</literal>).
  </para>

  <para>
   <indexterm>
    <primary>between</primary>
   </indexterm>
   In addition to the comparison operators, the special
   <token>BETWEEN</token> construct is available.
<synopsis>
<replaceable>a</replaceable> BETWEEN <replaceable>x</replaceable> AND <replaceable>y</replaceable>
</synopsis>
   is equivalent to
<synopsis>
<replaceable>a</replaceable> &gt;= <replaceable>x</replaceable> AND <replaceable>a</replaceable> &lt;= <replaceable>y</replaceable>
</synopsis>
   Similarly,
<synopsis>
<replaceable>a</replaceable> NOT BETWEEN <replaceable>x</replaceable> AND <replaceable>y</replaceable>
</synopsis>
   is equivalent to
<synopsis>
<replaceable>a</replaceable> &lt; <replaceable>x</replaceable> OR <replaceable>a</replaceable> &gt; <replaceable>y</replaceable>
</synopsis>
   There is no difference between the two respective forms apart from
   the <acronym>CPU</acronym> cycles required to rewrite the first one
   into the second one internally.
  </para>

  <para>
   To check whether a value is or is not NULL, use the constructs
<synopsis>
<replaceable>expression</replaceable> IS NULL
<replaceable>expression</replaceable> IS NOT NULL
</synopsis>
   Do <emphasis>not</emphasis> use
   <literal><replaceable>expression</replaceable> = NULL</literal>
   because NULL is not <quote>equal to</quote> NULL.  (NULL represents
   an unknown value, so it is not known whether two unknown values are
   equal.)  <productname>Postgres</productname> presently converts
   <literal>x = NULL</literal> clauses to <literal>x IS NULL</literal> to
   allow some broken client applications (such as
   <productname>Microsoft Access</productname>) to work, but this may
   be discontinued in a future release.
  </para>

  <para>
   Boolean values can be tested using the constructs
<synopsis>
<replaceable>expression</replaceable> IS TRUE
<replaceable>expression</replaceable> IS NOT TRUE
<replaceable>expression</replaceable> IS FALSE
<replaceable>expression</replaceable> IS NOT FALSE
<replaceable>expression</replaceable> IS UNKNOWN
<replaceable>expression</replaceable> IS NOT UNKNOWN
</synopsis>
   These are similar to <literal>IS NULL</literal> in that they will
   always return TRUE or FALSE, never NULL, even when the operand is NULL.
   A NULL input is treated as the logical value UNKNOWN.
  </para>
 </sect1>


 <sect1 id="functions-math">
  <title>Mathematical Functions and Operators</title>

  <table>
   <title>Mathematical Operators</TITLE>

   <tgroup cols="4">
    <thead>
     <row>
      <entry>Name</entry>
      <entry>Description</entry>
      <entry>Example</entry>
      <entry>Result</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry> <literal>+</literal> </entry>
      <entry>Addition</entry>
      <entry>2 + 3</entry>
      <entry>5</entry>
     </row>

     <row>
      <entry> <literal>-</literal> </entry>
      <entry>Subtraction</entry>
      <entry>2 - 3</entry>
      <entry>-1</entry>
     </row>

     <row>
      <entry> <literal>*</literal> </entry>
      <entry>Multiplication</entry>
      <entry>2 * 3</entry>
      <entry>6</entry>
     </row>

     <row>
      <entry> <literal>/</literal> </entry>
      <entry>Division (integer division truncates results)</entry>
      <entry>4 / 2</entry>
      <entry>2</entry>
     </row>

     <row>
      <entry> <literal>%</literal> </entry>
      <entry>Modulo (remainder)</entry>
      <entry>5 % 4</entry>
      <entry>1</entry>
     </row>

     <row>
      <entry> <literal>^</literal> </entry>
      <entry>Exponentiation</entry>
      <entry>2.0 ^ 3.0</entry>
      <entry>8.0</entry>
     </row>

     <row>
      <entry> <literal>|/</literal> </entry>
      <entry>Square root</entry>
      <entry>|/ 25.0</entry>
      <entry>5.0</entry>
     </row>

     <row>
      <entry> <literal>||/</literal> </entry>
      <entry>Cube root</entry>
      <entry>||/ 27.0</entry>
      <entry>3</entry>
     </row>

     <row>
      <entry> <literal>!</literal> </entry>
      <entry>Factorial</entry>
      <entry>5 !</entry>
      <entry>120</entry>
     </row>

     <row>
      <entry> <literal>!!</literal> </entry>
      <entry>Factorial (prefix operator)</entry>
      <entry>!! 5</entry>
      <entry>120</entry>
     </row>

     <row>
      <entry> <literal>@</literal> </entry>
      <entry>Absolute value</entry>
      <entry>@ -5.0</entry>
      <entry>5.0</entry>
     </row>

     <row>
      <entry> <literal>&amp;</literal> </entry>
      <entry>Binary AND</entry>
      <entry>91 & 15</entry>
      <entry>11</entry>
     </row>

     <row>
      <entry> <literal>|</literal> </entry>
      <entry>Binary OR</entry>
      <entry>32 | 3</entry>
      <entry>35</entry>
     </row>

     <row>
      <entry> <literal>#</literal> </entry>
      <entry>Binary XOR</entry>
      <entry>17 # 5</entry>
      <entry>20</entry>
     </row>

     <row>
      <entry> <literal>~</literal> </entry>
      <entry>Binary NOT</entry>
      <entry>~1</entry>
      <entry>-2</entry>
     </row>

     <row>
      <entry> &lt;&lt; </entry>
      <entry>Binary shift left</entry>
      <entry>1 &lt;&lt; 4</entry>
      <entry>16</entry>
     </row>

     <row>
      <entry> &gt;&gt; </entry>
      <entry>Binary shift right</entry>
      <entry>8 &gt;&gt; 2</entry>
      <entry>2</entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   The <quote>binary</quote> operators are also available for the bit
   string types <type>BIT</type> and <type>BIT VARYING</type>.

   <table>
    <title>Bit String Binary Operators</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Example</entry>
       <entry>Result</entry>
      </row>
     </thead>

     <tbody>
      <row>
       <entry>B'10001' & B'01101'</entry>
       <entry>00001</entry>
      </row>
      <row>
       <entry>B'10001' | B'01101'</entry>
       <entry>11101</entry>
      </row>
      <row>
       <entry>B'10001' # B'01101'</entry>
       <entry>11110</entry>
      </row>
      <row>
       <entry>~ B'10001'</entry>
       <entry>01110</entry>
      </row>
      <row>
       <entry>B'10001' << 3</entry>
       <entry>01000</entry>
      </row>
      <row>
       <entry>B'10001' >> 2</entry>
       <entry>00100</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   Bit string arguments to <literal>&</literal>, <literal>|</literal>,
   and <literal>#</literal> must be of equal length.  When bit
   shifting, the original length of the string is preserved, as shown
   here.
  </para>

  <table tocentry="1">
   <title>Mathematical Functions</title>
   <tgroup cols="5">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
      <entry>Example</entry>
      <entry>Result</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>abs(<replaceable>x</replaceable>)</entry>
      <entry>(same as x)</entry>
      <entry>absolute value</entry>
      <entry>abs(-17.4)</entry>
      <entry>17.4</entry>
     </row>

     <row>
      <entry>cbrt(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>cube root</entry>
      <entry>cbrt(27.0)</entry>
      <entry>3.0</entry>
     </row>

     <row>
      <entry>ceil(<type>numeric</type>)</entry>
      <entry><type>numeric</type></entry>
      <entry>smallest integer not less than argument</entry>
      <entry>ceil(-42.8)</entry>
      <entry>-42</entry>
     </row>

     <row>
      <entry>degrees(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>radians to degrees</entry>
      <entry>degrees(0.5)</entry>
      <entry>28.6478897565412</entry>
     </row>

     <row>
      <entry>exp(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>exponential</entry>
      <entry>exp(1.0)</entry>
      <entry>2.71828182845905</entry>
     </row>

     <row>
      <entry>floor(<type>numeric</type>)</entry>
      <entry><type>numeric</type></entry>
      <entry>largest integer not greater than argument</entry>
      <entry>floor(-42.8)</entry>
      <entry>-43</entry>
     </row>

     <row>
      <entry>ln(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>natural logarithm</entry>
      <entry>ln(2.0)</entry>
      <entry>0.693147180559945</entry>
     </row>

     <row>
      <entry>log(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>base 10 logarithm</entry>
      <entry>log(100.0)</entry>
      <entry>2.0</entry>
     </row>

     <row>
      <entry>log(<parameter>b</parameter> <type>numeric</type>,
       <parameter>x</parameter> <type>numeric</type>)</entry>
      <entry><type>numeric</type></entry>
      <entry>logarithm to base <parameter>b</parameter></entry>
      <entry>log(2.0, 64.0)</entry>
      <entry>6.0</entry>
     </row>

     <row>
      <entry>mod(<parameter>y</parameter>, <parameter>x</parameter>)</entry>
      <entry>(same as argument types)</entry>
      <entry>remainder of <parameter>y</parameter>/<parameter>x</parameter></entry>
      <entry>mod(9,4)</entry>
      <entry>1</entry>
     </row>

     <row>
      <entry>pi()</entry>
      <entry><type>dp</type></entry>
      <entry><quote>Pi</quote> constant</entry>
      <entry>pi()</entry>
      <entry>3.14159265358979</entry>
     </row>

     <row>
      <entry>pow(<parameter>e</parameter> <type>dp</type>,
       <parameter>n</parameter> <type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>raise a number to exponent <parameter>e</parameter></entry>
      <entry>pow(9.0, 3.0)</entry>
      <entry>729.0</entry>
     </row>

     <row>
      <entry>radians(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>degrees to radians</entry>
      <entry>radians(45.0)</entry>
      <entry>0.785398163397448</entry>
     </row>

     <row>
      <entry>random()</entry>
      <entry><type>dp</type></entry>
      <entry>value between 0.0 to 1.0</entry>
      <entry>random()</entry>
      <entry></entry>
     </row>

     <row>
      <entry>round(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>round to nearest integer</entry>
      <entry>round(42.4)</entry>
      <entry>42</entry>
     </row>

     <row>
      <entry>round(<parameter>v</parameter> <type>numeric</type>, <parameter>s</parameter> <type>integer</type>)</entry>
      <entry><type>numeric</type></entry>
      <entry>round to <parameter>s</parameter> decimal places</entry>
      <entry>round(42.4382, 2)</entry>
      <entry>42.44</entry>
     </row>
<!--
     <row>
      <entry>setseed(<replaceable>new-seed</replaceable>)</entry>
      <entry>set seed for subsequent random() calls</entry>
      <entry>setseed(0.54823)</entry>
      <entry></entry>
     </row>
-->
     <row>
      <entry>sqrt(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>square root</entry>
      <entry>sqrt(2.0)</entry>
      <entry>1.4142135623731</entry>
     </row>

     <row>
      <entry>trunc(<type>dp</type>)</entry>
      <entry><type>dp</type></entry>
      <entry>truncate toward zero</entry>
      <entry>trunc(42.8)</entry>
      <entry>42</entry>
     </row>

     <row>
      <entry>trunc(<type>numeric</type>, <parameter>s</parameter> <type>integer</type>)</entry>
      <entry><type>numeric</type></entry>
      <entry>truncate to <parameter>s</parameter> decimal places</entry>
      <entry>round(42.4382, 2)</entry>
      <entry>42.43</entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   In the table above, "<literal>dp</literal>" indicates <type>double precision</type>.
   The functions <function>exp</function>, <function>ln</function>,
   <function>log</function>, <function>pow</function>,
   <function>round</function> (1 argument), <function>sqrt</function>,
   and <function>trunc</function> (1 argument) are also available for
   the type <type>numeric</type> in place of <type>double
   precision</type>.
   Functions returning a <type>numeric</type> result take
   <type>numeric</type> input arguments, unless otherwise specified.
   Many of these functions are implemented on top
   of the host system's C library and behavior in boundary cases could
   therefore vary depending on the operating system.
  </para>

  <table>
   <title>Trigonometric Functions</title>

   <tgroup cols="2">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Description</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>acos(<replaceable>x</replaceable>)</entry>
      <entry>inverse cosine</entry>
     </row>

     <row>
      <entry>asin(<replaceable>x</replaceable>)</entry>
      <entry>inverse sine</entry>
     </row>

     <row>
      <entry>atan(<replaceable>x</replaceable>)</entry>
      <entry>inverse tangent</entry>
     </row>

     <row>
      <entry>atan2(<replaceable>x</replaceable>, <replaceable>y</replaceable>)</entry>
      <entry>inverse tangent of <replaceable>y</replaceable>/<replaceable>x</replaceable></entry>
     </row>

     <row>
      <entry>cos(<replaceable>x</replaceable>)</entry>
      <entry>cosine</entry>
     </row>

     <row>
      <entry>cot(<replaceable>x</replaceable>)</entry>
      <entry>cotangent</entry>
     </row>

     <row>
      <entry>sin(<replaceable>x</replaceable>)</entry>
      <entry>sine</entry>
     </row>

     <row>
      <entry>tan(<replaceable>x</replaceable>)</entry>
      <entry>tangent</entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   All trigonometric functions have arguments and return values of
   type <type>double precision</type>.
  </para>

 </sect1>


 <sect1 id="functions-string">
  <title>String Functions and Operators</title>

  <para>
   This section describes functions and operators for examining and
   manipulating string values.  Strings in this context include values
   of all the types <type>CHARACTER</type>, <type>CHARACTER
   VARYING</type>, and <type>TEXT</type>.  Unless otherwise noted, all
   of the functions listed below work on all of these types, but be
   wary of potential effects of the automatic padding when using the
   <type>CHARACTER</type> type.  Generally the functions described
   here also work on data of non-string types by converting that data
   to a string representation first.  Some functions also exist
   natively for bit string types.
  </para>

  <para>
   <acronym>SQL</acronym> defines some string functions with a special syntax where
   certain keywords rather than commas are used to separate the
   arguments.  Details are in <xref linkend="functions-string-sql">.
   These functions are also implemented using the regular syntax for
   function invocation.  (See <xref linkend="functions-string-other">.)
  </para>

  <table id="functions-string-sql">
   <title><acronym>SQL</acronym> String Functions and Operators</title>
   <tgroup cols="5">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
      <entry>Example</entry>
      <entry>Result</entry>  
     </row>
    </thead>

    <tbody>
     <row>
      <entry> <parameter>string</parameter> <literal>||</literal> <parameter>string</parameter> </entry>
      <entry> <type>text</type> </entry>
      <entry>
       string concatenation
       <indexterm>
        <primary>character strings</primary>
	<secondary>concatenation</secondary>
       </indexterm>
      </entry>
      <entry>'Postgre' || 'SQL'</entry>
      <entry>PostgreSQL</entry>
     </row>

     <row>
      <entry>bit_length(<parameter>string</parameter>)</entry>
      <entry><type>integer</type></entry>
      <entry>number of bits in string</entry>
      <entry>bit_length('jose')</entry>
      <entry>32</entry>
     </row>

     <row>
      <entry>char_length(<parameter>string</parameter>) or character_length(<parameter>string</parameter>)</entry>
      <entry><type>integer</type></entry>
      <entry>
       length of string
       <indexterm>
        <primary>character strings</primary>
	<secondary>length</secondary>
       </indexterm>
       <indexterm>
        <primary>length</primary>
	<secondary>character strings</secondary>
	<see>character strings, length</see>
       </indexterm>
      </entry>
      <entry>char_length('jose')</entry>
      <entry>4</entry>
     </row>

     <row>
      <entry>lower(<parameter>string</parameter>)</entry>
      <entry><type>text</type></entry>
      <entry>Convert string to lower case.</entry>
      <entry>lower('TOM')</entry>
      <entry>tom</entry>
     </row>

     <row>
      <entry>octet_length(<parameter>string</parameter>)</entry>
      <entry><type>integer</type></entry>
      <entry>number of bytes in string</entry>
      <entry>octet_length('jose')</entry>
      <entry>4</entry>
     </row>

     <row>
      <entry>position(<parameter>substring</parameter> in <parameter>string</parameter>)</entry>
      <entry><type>integer</type></entry>
      <entry>location of specified substring</entry>
      <entry>position('om' in 'Thomas')</entry>
      <entry>3</entry>
     </row>

     <row>
      <entry>substring(<parameter>string</parameter> <optional>from <type>integer</type></optional> <optional>for <type>integer</type></optional>)</entry>
      <entry><type>text</type></entry>
      <entry>
       extract substring
       <indexterm>
        <primary>substring</primary>
       </indexterm>
      </entry>
      <entry>substring('Thomas' from 2 for 3)</entry>
      <entry>hom</entry>
     </row>

     <row>
      <entry>
       trim(<optional>leading | trailing | both</optional>
       <optional><parameter>characters</parameter></optional> from
       <parameter>string</parameter>)
      </entry>
      <entry><type>text</type></entry>
      <entry>
       Removes the longest string containing only the
       <parameter>characters</parameter> (a space by default) from the
       beginning/end/both ends of the <parameter>string</parameter>.
      </entry>
      <entry>trim(both 'x' from 'xTomx')</entry>
      <entry>Tom</entry>
     </row>

     <row>
      <entry>upper(<parameter>string</parameter>)</entry>
      <entry><type>text</type></entry>
      <entry>Convert string to upper case.</entry>
      <entry>upper('tom')</entry>
      <entry>TOM</entry>
     </row>
    </tbody>
   </tgroup>
  </table>

  <para>
   Additional string manipulation functions are available and are
   listed below.  Some of them are used internally to implement the
   <acronym>SQL</acronym> string functions listed above.
  </para>

  <table id="functions-string-other">
   <title>Other String Functions</title>
   <tgroup cols="5">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Return Type</entry>
      <entry>Description</entry>
      <entry>Example</entry>
      <entry>Result</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>ascii(<type>text</type>)</entry>
      <entry>integer</entry>
      <entry>Returns the <acronym>ASCII</acronym> code of the first character of the argument.</entry>
      <entry>ascii('x')</entry>
      <entry>120</entry>
     </row>

     <row>
      <entry>btrim(<parameter>string</parameter> <type>text</type>, <parameter>trim</parameter> <type>text</type>)</entry>
      <entry><type>text</type></entry>
      <entry>
       Remove (trim) the longest string consisting only of characters
       in <parameter>trim</parameter> from the start and end of
       <parameter>string</parameter>.
      </entry>
      <entry>btrim('xyxtrimyyx','xy')</entry>
      <entry>trim</entry>
     </row>

     <row>
      <entry>chr(<type>integer</type>)</entry>
      <entry><type>text</type></entry>
      <entry>Returns the character with the given <acronym>ASCII</acronym> code.</entry>
      <entry>chr(65)</entry>
      <entry>A</entry>
     </row>

     <row>
      <entry>convert(<parameter>string</parameter> <type>text</type>,
      [<parameter>src_encoding</parameter> <type>name</type>,]
      <parameter>dest_encoding</parameter> <type>name</type>)</entry>
      <entry><type>text</type></entry>
      <entry>Converts string using <parameter>dest_encoding</parameter>.
      The original encoding is specified by <parameter>src_encoding</parameter>.
      If <parameter>src_encoding</parameter> is omitted, database encoding
      is assumed.
      </entry>
      <entry>convert('text_in_unicode','UNICODE','LATIN1')</entry>
      <entry>text_in_unicode (represented in ISO-8859-1)</entry>
     </row>

     <row>
      <entry>initcap(<type>text</type>)</entry>
      <entry><type>text</type></entry>
      <entry>Converts first letter of each word (whitespace separated) to upper case.</entry>
      <entry>initcap('hi thomas')</entry>
      <entry>Hi Thomas</entry>
     </row>

     <row>
      <entry>
       lpad(<parameter>string</parameter> <type>text</type>,
       <parameter>length</parameter> <type>integer</type>
       <optional>, <parameter>fill</parameter> <type>text</type></optional>)
      </entry>
      <entry>text</entry>
      <entry>
       Fills up the <parameter>string</parameter> to length
       <parameter>length</parameter> by prepending the characters
       <parameter>fill</parameter> (a space by default).  If the
       <parameter>string</parameter> is already longer than
       <parameter>length</parameter> then it is truncated (on the
       right).
      </entry>
      <entry>lpad('hi', 5, 'xy')</entry>
      <entry>xyxhi</entry>
     </row>

     <row>
      <entry>ltrim(<parameter>string</parameter> <type>text</type>, <parameter>trim</parameter> <type>text</type>)</entry>
      <entry><type>text</type></entry>
      <entry>
       Removes the longest string containing only characters from
       <parameter>trim</parameter> from the start of the string.
      </entry>
      <entry>ltrim('zzzytrim','xyz')</entry>
      <entry>trim</entry>
     </row>

     <row>
      <entry>repeat(<type>text</type>, <type>integer</type>)</entry>
      <entry><type>text</type></entry>
      <entry>Repeat text a number of times.</entry>
      <entry>repeat('Pg', 4)</entry>
      <entry>PgPgPgPg</entry>
     </row>

     <row>
      <entry>
       rpad(<parameter>string</parameter> <type>text</type>,
       <parameter>length</parameter> <type>integer</type>
       <optional>, <parameter>fill</parameter> <type>text</type></optional>)
      </entry>
      <entry><type>text</type></entry>
      <entry>
       Fills up the <parameter>string</parameter> to length
       <parameter>length</parameter> by appending the characters
       <parameter>fill</parameter> (a space by default).  If the
       <parameter>string</parameter> is already longer than
       <parameter>length</parameter> then it is truncated.
      </entry>
      <entry>rpad('hi', 5, 'xy')</entry>
      <entry>hixyx</entry>
     </row>

     <row>
      <entry>rtrim(<parameter>string</parameter> text, <parameter>trim</parameter> text)</entry>
      <entry><type>text</type></entry>
      <entry>
       Removes the longest string containing only characters from
       <parameter>trim</parameter> from the end of the string.
      </entry>
      <entry>rtrim('trimxxxx','x')</entry>
      <entry>trim</entry>
     </row>

     <row>
      <entry>strpos(<parameter>string</parameter>, <parameter>substring</parameter>)</entry>
      <entry><type>text</type></entry>
      <entry>
       Locates specified substring. (same as
       <literal>position(<parameter>substring</parameter> in
       <parameter>string</parameter>)</literal>, but note the reversed
       argument order)
      </entry>
      <entry>strpos('high','ig')</entry>
      <entry>2</entry>
     </row>

     <row>
      <entry>substr(<parameter>string</parameter>, <parameter>from</parameter> <optional>, <parameter>count</parameter></optional>)</entry>
      <entry><type>text</type></entry>
      <entry>
       Extracts specified substring. (same as <literal>substring(<parameter>string</parameter> from <parameter>from</parameter> for <parameter>count</parameter>)</literal>)
      </entry>
      <entry>substr('alphabet', 3, 2)</entry>
      <entry>ph</entry>
     </row>

     <row>
      <entry>to_ascii(<type>text</type> <optional>, <parameter>encoding</parameter></optional>)</entry>
      <entry><type>text</type></entry>
      <entry>Converts text from multibyte encoding to <acronym>ASCII</acronym>.</entry>
      <entry>to_ascii('Karel')</entry>
      <entry>Karel</entry>
     </row>

     <row>
      <entry>
       translate(<parameter>string</parameter> <type>text</type>,
       <parameter>from</parameter> <type>text</type>,
       <parameter>to</parameter> <type>text</type>)
      </entry>
      <entry><type>text</type></entry>
      <entry>
       Any character in <parameter>string</parameter> that matches a
       character in the <parameter>from</parameter> set is replaced by
       the corresponding character in the <parameter>to</parameter>
       set.
      </entry>
      <entry>translate('12345', '14', 'ax')</entry>
      <entry>a23x5</entry>
     </row>       
     
     <row>
      <entry>
       encode(<parameter>data</parameter> <type>bytea</type>,
              <parameter>type</parameter> <type>text</type>)
      </entry>
      <entry><type>text</type></entry>
      <entry>
       Encodes binary data to ascii-only representation.  Supported
       types are: 'base64', 'hex'.
      </entry>
      <entry>encode('123\\000\\001', 'base64')</entry>
      <entry>MTIzAAE=</entry>
     </row>       

     <row>
      <entry>
       decode(<parameter>string</parameter> <type>text</type>,
              <parameter>type</parameter> <type>text</type>)
      </entry>
      <entry><type>bytea</type></entry>
      <entry>
       Decodes binary data from <parameter>string</parameter> previously 
       encoded with encode().  Parameter type is same as in encode().
      </entry>
      <entry>decode('MTIzAAE=', 'base64')</entry>
      <entry>123\000\001</entry>
     </row>       

    </tbody>
   </tgroup>
  </table>

  <para>
   The <function>to_ascii</function> function supports conversion from
   LATIN1, LATIN2, WIN1250 (CP1250) only.
  </para>
 </sect1>


 <sect1 id="functions-matching">
  <title>Pattern Matching</title>

  <para>
   There are two separate approaches to pattern matching provided by
   <productname>Postgres</productname>:  the <acronym>SQL</acronym>
   <function>LIKE</function> operator and
   <acronym>POSIX</acronym>-style regular expressions.
  </para>

  <tip>
   <para>
    If you have pattern matching needs that go beyond this, or want to
    make pattern-driven substitutions or translations, consider
    writing a user-defined function in Perl or Tcl.
   </para>
  </tip>

  <sect2 id="functions-like">
   <title>Pattern Matching with <function>LIKE</function></title>

   <indexterm>
    <primary>like</primary>
   </indexterm>

<synopsis>
<replaceable>string</replaceable> LIKE <replaceable>pattern</replaceable> <optional> ESCAPE <replaceable>escape-character</replaceable> </optional>
<replaceable>string</replaceable> NOT LIKE <replaceable>pattern</replaceable> <optional> ESCAPE <replaceable>escape-character</replaceable> </optional>
</synopsis>

   <para>
    Every <replaceable>pattern</replaceable> defines a set of strings.
    The <function>LIKE</function> expression returns true if the
    <replaceable>string</replaceable> is contained in the set of
    strings represented by <replaceable>pattern</replaceable>.  (As
    expected, the <function>NOT LIKE</function> expression returns
    false if <function>LIKE</function> returns true, and vice versa.
    An equivalent expression is <literal>NOT
    (<replaceable>string</replaceable> LIKE
    <replaceable>pattern</replaceable>)</literal>.)
   </para>

   <para>
    If <replaceable>pattern</replaceable> does not contain percent
    signs or underscore, then the pattern only represents the string
    itself; in that case <function>LIKE</function> acts like the
    equals operator.  An underscore (<literal>_</literal>) in
    <replaceable>pattern</replaceable> stands for (matches) any single
    character; a percent sign (<literal>%</literal>) matches any string
    of zero or more characters.
   </para>

   <informalexample>
    <para>
     Some examples:
<programlisting>
'abc' LIKE 'abc'    <lineannotation>true</lineannotation>
'abc' LIKE 'a%'     <lineannotation>true</lineannotation>
'abc' LIKE '_b_'    <lineannotation>true</lineannotation>
'abc' LIKE 'c'      <lineannotation>false</lineannotation>
</programlisting>
    </para>
   </informalexample>

   <para>
    <function>LIKE</function> pattern matches always cover the entire
    string.  To match a pattern anywhere within a string, the
    pattern must therefore start and end with a percent sign.
   </para>

   <para>
    To match a literal underscore or percent sign without matching
    other characters, the respective character in
    <replaceable>pattern</replaceable> must be 
    preceded by the escape character.  The default escape
    character is the backslash but a different one may be selected by
    using the <literal>ESCAPE</literal> clause.  To match the escape
    character itself, write two escape characters.
   </para>

   <para>
    Note that the backslash already has a special meaning in string
    literals, so to write a pattern constant that contains a backslash
    you must write two backslashes in the query.  You can avoid this by
    selecting a different escape character with <literal>ESCAPE</literal>.
   </para>

   <para>
    The keyword <token>ILIKE</token> can be used instead of
    <token>LIKE</token> to make the match case insensitive according
    to the active locale.  This is not in the <acronym>SQL</acronym> standard but is a
    <productname>Postgres</productname> extension.
   </para>

   <para>
    The operator <literal>~~</literal> is equivalent to
    <function>LIKE</function>, and <literal>~~*</literal> corresponds to
    <function>ILIKE</function>.  There are also
    <literal>!~~</literal> and <literal>!~~*</literal> operators that
    represent <function>NOT LIKE</function> and <function>NOT
    ILIKE</function>.  All of these are also
    <productname>Postgres</productname>-specific.
   </para>
  </sect2>


  <sect2 id="functions-regexp">
   <title><acronym>POSIX</acronym> Regular Expressions</title>

   <indexterm zone="functions-regexp">
    <primary>regular expressions</primary>
    <seealso>pattern matching</seealso>
   </indexterm>

   <table>
    <title>Regular Expression Match Operators</title>

    <tgroup cols="3">
     <thead>
      <row>
       <entry>Operator</entry>
       <entry>Description</entry>
       <entry>Example</entry>
      </row>
     </thead>

     <tbody>
       <ROW>
	<ENTRY> <literal>~</literal> </ENTRY>
	<ENTRY>Matches regular expression, case sensitive</ENTRY>
	<ENTRY>'thomas' ~ '.*thomas.*'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> <literal>~*</literal> </ENTRY>
	<ENTRY>Matches regular expression, case insensitive</ENTRY>
	<ENTRY>'thomas' ~* '.*Thomas.*'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> <literal>!~</literal> </ENTRY>
	<ENTRY>Does not match regular expression, case sensitive</ENTRY>
	<ENTRY>'thomas' !~ '.*Thomas.*'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> <literal>!~*</literal> </ENTRY>
	<ENTRY>Does not match regular expression, case insensitive</ENTRY>
	<ENTRY>'thomas' !~* '.*vadim.*'</ENTRY>
       </ROW>
     </tbody>
    </tgroup>
   </table>

   <para>
    <acronym>POSIX</acronym> regular expressions provide a more powerful means for
    pattern matching than the <function>LIKE</function> function.
    Many Unix tools such as <command>egrep</command>,
    <command>sed</command>, or <command>awk</command> use a pattern
    matching language that is similar to the one described here.
   </para>

   <para>
    A regular expression is a character sequence that is an
    abbreviated definition of a set of strings (a <firstterm>regular
    set</firstterm>).  A string is said to match a regular expression
    if it is a member of the regular set described by the regular
    expression.  As with <function>LIKE</function>, pattern characters
    match string characters exactly unless they are special characters
    in the regular expression language --- but regular expressions use
    different special characters than <function>LIKE</function> does.
    Unlike <function>LIKE</function> patterns, a
    regular expression is allowed to match anywhere within a string, unless
    the regular expression is explicitly anchored to the beginning or
    end of the string.
   </para>


<!-- derived from the re_format.7 man page -->
   <para>
    Regular expressions (<quote>RE</quote>s), as defined in <acronym>POSIX</acronym>
    1003.2, come in two forms: modern REs (roughly those of
    <command>egrep</command>; 1003.2 calls these
    <quote>extended</quote> REs) and obsolete REs (roughly those of
    <command>ed</command>; 1003.2 <quote>basic</quote> REs).
    <productname>Postgres</productname> implements the modern form.
   </para>

   <para>
    A (modern) RE is one or more non-empty
    <firstterm>branches</firstterm>, separated by
    <literal>|</literal>.  It matches anything that matches one of the
    branches.
   </para>

   <para>
    A branch is one or more <firstterm>pieces</firstterm>,
    concatenated.  It matches a match for the first, followed by a
    match for the second, etc.
   </para>

   <para>
    A piece is an <firstterm>atom</firstterm> possibly followed by a
    single <literal>*</literal>, <literal>+</literal>,
    <literal>?</literal>, or <firstterm>bound</firstterm>.  An atom
    followed by <literal>*</literal> matches a sequence of 0 or more
    matches of the atom.  An atom followed by <literal>+</literal>
    matches a sequence of 1 or more matches of the atom.  An atom
    followed by <literal>?</literal> matches a sequence of 0 or 1
    matches of the atom.
   </para>

   <para>
    A <firstterm>bound</firstterm> is <literal>{</literal> followed by
    an unsigned decimal integer, possibly followed by
    <literal>,</literal> possibly followed by another unsigned decimal
    integer, always followed by <literal>}</literal>.  The integers
    must lie between 0 and <symbol>RE_DUP_MAX</symbol> (255)
    inclusive, and if there are two of them, the first may not exceed
    the second.  An atom followed by a bound containing one integer
    <replaceable>i</replaceable> and no comma matches a sequence of
    exactly <replaceable>i</replaceable> matches of the atom.  An atom
    followed by a bound containing one integer
    <replaceable>i</replaceable> and a comma matches a sequence of
    <replaceable>i</replaceable> or more matches of the atom.  An atom
    followed by a bound containing two integers
    <replaceable>i</replaceable> and <replaceable>j</replaceable>
    matches a sequence of <replaceable>i</replaceable> through
    <replaceable>j</replaceable> (inclusive) matches of the atom.
   </para>

   <note>
    <para>
     A repetition operator (<literal>?</literal>,
     <literal>*</literal>, <literal>+</literal>, or bounds) cannot
     follow another repetition operator.  A repetition operator cannot
     begin an expression or subexpression or follow
     <literal>^</literal> or <literal>|</literal>.
    </para>
   </note>

   <para>
    An <firstterm>atom</firstterm> is a regular expression enclosed in
    <literal>()</literal> (matching a match for the regular
    expression), an empty set of <literal>()</literal> (matching the
    null string), a <firstterm>bracket expression</firstterm> (see
    below), <literal>.</literal> (matching any single character),
    <literal>^</literal> (matching the null string at the beginning of the
    input string), <literal>$</literal> (matching the null string at the end
    of the input string), a <literal>\</literal> followed by one of the
    characters <literal>^.[$()|*+?{\</literal> (matching that
    character taken as an ordinary character), a <literal>\</literal>
    followed by any other character (matching that character taken as
    an ordinary character, as if the <literal>\</literal> had not been
    present), or a single character with no other significance
    (matching that character).  A <literal>{</literal> followed by a
    character other than a digit is an ordinary character, not the
    beginning of a bound.  It is illegal to end an RE with
    <literal>\</literal>.
   </para>

   <para>
    Note that the backslash (<literal>\</literal>) already has a special
    meaning in string
    literals, so to write a pattern constant that contains a backslash
    you must write two backslashes in the query.
   </para>

   <para>
    A <firstterm>bracket expression</firstterm> is a list of
    characters enclosed in <literal>[]</literal>.  It normally matches
    any single character from the list (but see below).  If the list
    begins with <literal>^</literal>, it matches any single character
    (but see below) not from the rest of the list.  If two characters
    in the list are separated by <literal>-</literal>, this is
    shorthand for the full range of characters between those two
    (inclusive) in the collating sequence,
    e.g. <literal>[0-9]</literal> in <acronym>ASCII</acronym> matches
    any decimal digit.  It is illegal for two ranges to share an
    endpoint, e.g.  <literal>a-c-e</literal>.  Ranges are very
    collating-sequence-dependent, and portable programs should avoid
    relying on them.
   </para>

   <para>
    To include a literal <literal>]</literal> in the list, make it the
    first character (following a possible <literal>^</literal>).  To
    include a literal <literal>-</literal>, make it the first or last
    character, or the second endpoint of a range.  To use a literal
    <literal>-</literal> as the first endpoint of a range, enclose it
    in <literal>[.</literal> and <literal>.]</literal> to make it a
    collating element (see below).  With the exception of these and
    some combinations using <literal>[</literal> (see next
    paragraphs), all other special characters, including
    <literal>\</literal>, lose their special significance within a
    bracket expression.
   </para>

   <para>
    Within a bracket expression, a collating element (a character, a
    multi-character sequence that collates as if it were a single
    character, or a collating-sequence name for either) enclosed in
    <literal>[.</literal> and <literal>.]</literal> stands for the
    sequence of characters of that collating element.  The sequence is
    a single element of the bracket expression's list.  A bracket
    expression containing a multi-character collating element can thus
    match more than one character, e.g. if the collating sequence
    includes a <literal>ch</literal> collating element, then the RE
    <literal>[[.ch.]]*c</literal> matches the first five characters of
    <literal>chchcc</literal>.
   </para>

   <para>
    Within a bracket expression, a collating element enclosed in
    <literal>[=</literal> and <literal>=]</literal> is an equivalence
    class, standing for the sequences of characters of all collating
    elements equivalent to that one, including itself.  (If there are
    no other equivalent collating elements, the treatment is as if the
    enclosing delimiters were <literal>[.</literal> and
    <literal>.]</literal>.)  For example, if <literal>o</literal> and
    <literal>^</literal> are the members of an equivalence class, then
    <literal>[[=o=]]</literal>, <literal>[[=^=]]</literal>, and
    <literal>[o^]</literal> are all synonymous.  An equivalence class
    may not be an endpoint of a range.
   </para>

   <para>
    Within a bracket expression, the name of a character class
    enclosed in <literal>[:</literal> and <literal>:]</literal> stands
    for the list of all characters belonging to that class.  Standard
    character class names are: <literal>alnum</literal>,
    <literal>alpha</literal>, <literal>blank</literal>,
    <literal>cntrl</literal>, <literal>digit</literal>,
    <literal>graph</literal>, <literal>lower</literal>,
    <literal>print</literal>, <literal>punct</literal>,
    <literal>space</literal>, <literal>upper</literal>,
    <literal>xdigit</literal>.  These stand for the character classes
    defined in
    <citerefentry><refentrytitle>ctype</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    A locale may provide others.  A character class may not be used as
    an endpoint of a range.
   </para>

   <para>
    There are two special cases of bracket expressions:  the bracket
    expressions <literal>[[:<:]]</literal> and
    <literal>[[:>:]]</literal> match the null string at the beginning
    and end of a word respectively.  A word is defined as a sequence
    of word characters which is neither preceded nor followed by word
    characters.  A word character is an alnum character (as defined by
    <citerefentry><refentrytitle>ctype</refentrytitle><manvolnum>3</manvolnum></citerefentry>)
    or an underscore.  This is an extension, compatible with but not
    specified by POSIX 1003.2, and should be used with caution in
    software intended to be portable to other systems.
   </para>

   <para>
    In the event that an RE could match more than one substring of a
    given string, the RE matches the one starting earliest in the
    string.  If the RE could match more than one substring starting at
    that point, it matches the longest.  Subexpressions also match the
    longest possible substrings, subject to the constraint that the
    whole match be as long as possible, with subexpressions starting
    earlier in the RE taking priority over ones starting later.  Note
    that higher-level subexpressions thus take priority over their
    lower-level component subexpressions.
   </para>

   <para>
    Match lengths are measured in characters, not collating
    elements.  A null string is considered longer than no match at
    all.  For example, <literal>bb*</literal> matches the three middle
    characters of <literal>abbbc</literal>,
    <literal>(wee|week)(knights|nights)</literal> matches all ten
    characters of <literal>weeknights</literal>, when
    <literal>(.*).*</literal> is matched against
    <literal>abc</literal> the parenthesized subexpression matches all
    three characters, and when <literal>(a*)*</literal> is matched
    against <literal>bc</literal> both the whole RE and the
    parenthesized subexpression match the null string.
   </para>

   <para>
    If case-independent matching is specified, the effect is much as
    if all case distinctions had vanished from the alphabet.  When an
    alphabetic that exists in multiple cases appears as an ordinary
    character outside a bracket expression, it is effectively
    transformed into a bracket expression containing both cases,
    e.g. <literal>x</literal> becomes <literal>[xX]</literal>.  When
    it appears inside a bracket expression, all case counterparts of
    it are added to the bracket expression, so that (e.g.)
    <literal>[x]</literal> becomes <literal>[xX]</literal> and
    <literal>[^x]</literal> becomes <literal>[^xX]</literal>.
   </para>

   <para>
    There is no particular limit on the length of REs, except insofar
    as memory is limited.  Memory usage is approximately linear in RE
    size, and largely insensitive to RE complexity, except for bounded
    repetitions.  Bounded repetitions are implemented by macro
    expansion, which is costly in time and space if counts are large
    or bounded repetitions are nested.  An RE like, say,
    <literal>((((a{1,100}){1,100}){1,100}){1,100}){1,100}</literal>
    will (eventually) run almost any existing machine out of swap
    space.
    <footnote>
     <para>
      This was written in 1994, mind you.  The
      numbers have probably changed, but the problem
      persists.
     </para>
    </footnote>
   </para>
<!-- end re_format.7 man page -->
  </sect2>

 </sect1>


  <sect1 id="functions-formatting">
   <title>Formatting Functions</title>

   <indexterm zone="functions-formatting">
    <primary>formatting</primary>
   </indexterm>

   <note>
    <title>Author</title>
    <para>
     Written by Karel Zak (<email>zakkr@zf.jcu.cz</email>) on 2000-01-24
    </para>
   </note>

   <para>
    The <productname>Postgres</productname> formatting functions
    provide a powerful set of tools for converting various data types
    (date/time, integer, floating point, numeric) to formatted strings
    and for converting from formatted strings to specific data types.
    These functions all follow a common calling convention: the first
    argument is the value to be formatted and the second argument is a
    template that defines the output or input format.
   </para>

   <para>
    <table tocentry="1">
     <title>Formatting Functions</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Function</entry>
	<entry>Returns</entry>
	<entry>Description</entry>
	<entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>to_char(timestamp, text)</entry>
	<entry>text</entry>
	<entry>convert timestamp to string</entry>
	<entry>to_char(timestamp 'now','HH12:MI:SS')</entry>
       </row>
       <row>
	<entry>to_char(int, text)</entry>
	<entry>text</entry>
	<entry>convert int4/int8 to string</entry>
	<entry>to_char(125, '999')</entry>
       </row>
       <row>
	<entry>to_char(double precision, text)</entry>
	<entry>text</entry>
	<entry>convert real/double precision to string</entry>
	<entry>to_char(125.8, '999D9')</entry>
       </row>
       <row>
	<entry>to_char(numeric, text)</entry>
	<entry>text</entry>
	<entry>convert numeric to string</entry>
	<entry>to_char(numeric '-125.8', '999D99S')</entry>
       </row>
       <row>
	<entry>to_date(text, text)</entry>
	<entry>date</entry>
	<entry>convert string to date</entry>
	<entry>to_date('05 Dec 2000', 'DD Mon YYYY')</entry>
       </row>
       <row>
	<entry>to_timestamp(text, text)</entry>
	<entry>timestamp</entry>
	<entry>convert string to timestamp</entry>
	<entry>to_timestamp('05 Dec 2000', 'DD Mon YYYY')</entry>
       </row>
       <row>
	<entry>to_number(text, text)</entry>
	<entry>numeric</entry>
	<entry>convert string to numeric</entry>
	<entry>to_number('12,454.8-', '99G999D9S')</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <para>
    In an output template string, there are certain patterns that are
    recognized and replaced with appropriately-formatted data from the value
    to be formatted.  Any text that is not a template pattern is simply
    copied verbatim.  Similarly, in an input template string template patterns
    identify the parts of the input data string to be looked at and the
    values to be found there.
   </para>

   <para>
    <table tocentry="1">
     <title>Template patterns for date/time conversions</title>
     <tgroup cols="2">
      <thead>
       <row>
	<entry>Pattern</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>HH</entry>
	<entry>hour of day (01-12)</entry>
       </row>
       <row>
	<entry>HH12</entry>
	<entry>hour of day (01-12)</entry>
       </row>       
       <row>
	<entry>HH24</entry>
	<entry>hour of day (00-23)</entry>
       </row>       
       <row>
	<entry>MI</entry>
	<entry>minute (00-59)</entry>
       </row>   
       <row>
	<entry>SS</entry>
	<entry>second (00-59)</entry>
       </row>
       <row>
	<entry>SSSS</entry>
	<entry>seconds past midnight (0-86399)</entry>
       </row>
       <row>
	<entry>AM or A.M. or PM or P.M.</entry>
	<entry>meridian indicator (upper case)</entry>
       </row>
       <row>
	<entry>am or a.m. or pm or p.m.</entry>
	<entry>meridian indicator (lower case)</entry>
       </row>
       <row>
	<entry>Y,YYY</entry>
	<entry>year (4 and more digits) with comma</entry>
       </row>
       <row>
	<entry>YYYY</entry>
	<entry>year (4 and more digits)</entry>
       </row>
       <row>
	<entry>YYY</entry>
	<entry>last 3 digits of year</entry>
       </row>
       <row>
	<entry>YY</entry>
	<entry>last 2 digits of year</entry>
       </row>
       <row>
	<entry>Y</entry>
	<entry>last digit of year</entry>
       </row>
       <row>
	<entry>BC or B.C. or AD or A.D.</entry>
	<entry>year indicator (upper case)</entry>
       </row>
       <row>
	<entry>bc or b.c. or ad or a.d.</entry>
	<entry>year indicator (lower case)</entry>
       </row>
       <row>
	<entry>MONTH</entry>
	<entry>full upper case month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
	<entry>Month</entry>
	<entry>full mixed case month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
	<entry>month</entry>
	<entry>full lower case month name (blank-padded to 9 chars)</entry>
       </row>
       <row>
	<entry>MON</entry>
	<entry>abbreviated upper case month name (3 chars)</entry>
       </row>
       <row>
	<entry>Mon</entry>
	<entry>abbreviated mixed case month name (3 chars)</entry>
       </row>
       <row>
	<entry>mon</entry>
	<entry>abbreviated lower case month name (3 chars)</entry>
       </row>
       <row>
	<entry>MM</entry>
	<entry>month number (01-12)</entry>
       </row>
       <row>
	<entry>DAY</entry>
	<entry>full upper case day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
	<entry>Day</entry>
	<entry>full mixed case day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
	<entry>day</entry>
	<entry>full lower case day name (blank-padded to 9 chars)</entry>
       </row>
       <row>
	<entry>DY</entry>
	<entry>abbreviated upper case day name (3 chars)</entry>
       </row>
       <row>
	<entry>Dy</entry>
	<entry>abbreviated mixed case day name (3 chars)</entry>
       </row>
       <row>
	<entry>dy</entry>
	<entry>abbreviated lower case day name (3 chars)</entry>
       </row>
       <row>
	<entry>DDD</entry>
	<entry>day of year (001-366)</entry>
       </row>
       <row>
	<entry>DD</entry>
	<entry>day of month (01-31)</entry>
       </row>
       <row>
	<entry>D</entry>
	<entry>day of week (1-7; SUN=1)</entry>
       </row>
       <row>
	<entry>W</entry>
	<entry>week of month (1-5) where first week start on the first day of the month</entry>
       </row> 
       <row>
	<entry>WW</entry>
	<entry>week number of year (1-53) where first week start on the first day of the year</entry>
       </row>
       <row>
	<entry>IW</entry>
	<entry>ISO week number of year (The first Thursday of the new year is in week 1.)</entry>
       </row>
       <row>
	<entry>CC</entry>
	<entry>century (2 digits)</entry>
       </row>
       <row>
	<entry>J</entry>
	<entry>Julian Day (days since January 1, 4712 BC)</entry>
       </row>
       <row>
	<entry>Q</entry>
	<entry>quarter</entry>
       </row>
       <row>
	<entry>RM</entry>
	<entry>month in Roman Numerals (I-XII; I=January) - upper case</entry>
       </row>
       <row>
	<entry>rm</entry>
	<entry>month in Roman Numerals (I-XII; I=January) - lower case</entry>
       </row>
       <row>
	<entry>TZ</entry>
	<entry>timezone name - upper case</entry>
       </row>
       <row>
	<entry>tz</entry>
	<entry>timezone name - lower case</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <para>
    Certain modifiers may be applied to any template pattern to alter its
    behavior.  For example, <quote><literal>FMMonth</literal></quote>
    is the <quote><literal>Month</literal></quote> pattern with the
    <quote><literal>FM</literal></quote> prefix.
   </para>

   <para>
    <table tocentry="1">
     <title>Template pattern modifiers for date/time conversions</title>
     <tgroup cols="3">
      <thead>
       <row>
	<entry>Modifier</entry>
	<entry>Description</entry>
	<entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry><literal>FM</literal> prefix</entry>
	<entry>fill mode (suppress padding blanks and zeroes)</entry>
	<entry>FMMonth</entry>
       </row>
       <row>
	<entry><literal>TH</literal> suffix</entry>
	<entry>add upper-case ordinal number suffix</entry>
	<entry>DDTH</entry>
       </row>	
       <row>
	<entry><literal>th</literal> suffix</entry>
	<entry>add lower-case ordinal number suffix</entry>
	<entry>DDth</entry>
       </row>
       <row>
	<entry><literal>FX</literal> prefix</entry>
	<entry>FiXed format global option (see below)</entry>
	<entry>FX Month DD Day</entry>
       </row>	
       <row>
	<entry><literal>SP</literal> suffix</entry>
	<entry>spell mode (not yet implemented)</entry>
	<entry>DDSP</entry>
       </row>       
      </tbody>
     </tgroup>
    </table>
   </para>

   <para>
    Usage notes:

    <itemizedlist>
     <listitem>
      <para>
       <literal>FM</literal> suppresses leading zeroes or trailing blanks
       that would otherwise be added to make the output of a pattern be
       fixed-width.
      </para>
     </listitem>

     <listitem>
      <para>
       <function>to_timestamp</function> and <function>to_date</function>
       skip multiple blank spaces in the input string if the <literal>FX</literal> option 
       is not used. <literal>FX</literal> must be specified as the first item
       in the template; for example 
       <literal>to_timestamp('2000    JUN','YYYY MON')</literal> is right, but
       <literal>to_timestamp('2000    JUN','FXYYYY MON')</literal> returns an error,
       because <function>to_timestamp</function> expects one blank space only.
      </para>
     </listitem>

     <listitem>
      <para>
       If a backslash (<quote><literal>\</literal></quote>) is desired
       in a string constant, a double backslash
       (<quote><literal>\\</literal></quote>) must be entered; for
       example <literal>'\\HH\\MI\\SS'</literal>.  This is true for
       any string constant in <productname>Postgres</productname>.
      </para>
     </listitem>

     <listitem>
      <para>
       Ordinary text is allowed in <function>to_char</function>
       templates and will be output literally.  You can put a substring
       in double quotes to force it to be interpreted as literal text
       even if it contains pattern keywords.  For example, in
       <literal>'"Hello Year: "YYYY'</literal>, the <literal>YYYY</literal>
       will be replaced by year data, but the single <literal>Y</literal>
       will not be.
      </para>
     </listitem>

     <listitem>
      <para>
       If you want to have a double quote in the output you must
       precede it with a backslash, for example <literal>'\\"YYYY
       Month\\"'</literal>. <!-- "" font-lock sanity :-) -->
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>YYYY</literal> conversion from string to timestamp or
       date is restricted if you use a year with more than 4 digits. You must
       use some non-digit character or template after <literal>YYYY</literal>,
       otherwise the year is always interpreted as 4 digits. For example
       (with year 20000):
       <literal>to_date('200001131', 'YYYYMMDD')</literal> will be 
       interpreted as a 4-digit year; better is to use a non-digit 
       separator after the year, like
       <literal>to_date('20000-1131', 'YYYY-MMDD')</literal> or
       <literal>to_date('20000Nov31', 'YYYYMonDD')</literal>.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    <table tocentry="1">
     <title>Template patterns for numeric conversions</title>
     <tgroup cols="2">
      <thead>
       <row>
	<entry>Pattern</entry>
	<entry>Description</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>9</entry>
	<entry>value with the specified number of digits</entry>
       </row>
       <row>
	<entry>0</entry>
	<entry>value with leading zeros</entry>
       </row>
       <row>
	<entry>. (period)</entry>
	<entry>decimal point</entry>
       </row>       
       <row>
	<entry>, (comma)</entry>
	<entry>group (thousand) separator</entry>
       </row>
       <row>
	<entry>PR</entry>
	<entry>negative value in angle brackets</entry>
       </row>
       <row>
	<entry>S</entry>
	<entry>negative value with minus sign (uses locale)</entry>
       </row>
       <row>
	<entry>L</entry>
	<entry>currency symbol (uses locale)</entry>
       </row>
       <row>
	<entry>D</entry>
	<entry>decimal point (uses locale)</entry>
       </row>
       <row>
	<entry>G</entry>
	<entry>group separator (uses locale)</entry>
       </row>
       <row>
	<entry>MI</entry>
	<entry>minus sign in specified position (if number < 0)</entry>
       </row>
       <row>
	<entry>PL</entry>
	<entry>plus sign in specified position (if number > 0)</entry>
       </row>
       <row>
	<entry>SG</entry>
	<entry>plus/minus sign in specified position</entry>
       </row>
       <row>
	<entry>RN</entry>
	<entry>roman numeral (input between 1 and 3999)</entry>
       </row>
       <row>
	<entry>TH or th</entry>
	<entry>convert to ordinal number</entry>
       </row>
       <row>
	<entry>V</entry>
	<entry>shift <replaceable>n</replaceable> digits (see
	 notes)</entry>
       </row>
       <row>
	<entry>EEEE</entry>
	<entry>scientific numbers (not supported yet)</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <para>
    Usage notes:

    <itemizedlist>
     <listitem>
      <para>
       A sign formatted using 'SG', 'PL' or 'MI' is not an anchor in
       the number; for example,
       to_char(-12, 'S9999') produces <literal>'  -12'</literal>,
       but to_char(-12, 'MI9999') produces <literal>'-  12'</literal>.
       The Oracle implementation does not allow the use of
       <literal>MI</literal> ahead of <literal>9</literal>, but rather
       requires that <literal>9</literal> precede
       <literal>MI</literal>.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>9</literal> specifies a value with the same number of 
       digits as there are <literal>9</literal>s. If a digit is
       not available use blank space.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>TH</literal> does not convert values less than zero
       and does not convert decimal numbers.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>PL</literal>, <literal>SG</literal>, and
       <literal>TH</literal> are <productname>Postgres</productname>
       extensions. 
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>V</literal> effectively
       multiplies the input values by
       <literal>10^<replaceable>n</replaceable></literal>, where
       <replaceable>n</replaceable> is the number of digits following
       <literal>V</literal>. 
       <function>to_char</function> does not support the use of
       <literal>V</literal> combined with a decimal point.
       (E.g., <literal>99.9V99</literal> is not allowed.)
      </para>
     </listitem>
    </itemizedlist>
   </para>   

   <para>
    <table tocentry="1">
     <title><function>to_char</function> Examples</title>
     <tgroup cols="2">
      <thead>
       <row>
	<entry>Input</entry>
	<entry>Output</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry>to_char(now(),'Day, DD  HH12:MI:SS')</entry>
        <entry><literal>'Tuesday  , 06  05:39:18'</literal></entry>
       </row>
       <row>
        <entry>to_char(now(),'FMDay, FMDD  HH12:MI:SS')</entry>
        <entry><literal>'Tuesday, 6  05:39:18'</literal></entry>
       </row>          
       <row>
        <entry>to_char(-0.1,'99.99')</entry>
        <entry><literal>' -.10'</literal></entry>
       </row>
       <row>
        <entry>to_char(-0.1,'FM9.99')</entry>
        <entry><literal>'-.1'</literal></entry>
       </row>
       <row>
        <entry>to_char(0.1,'0.9')</entry>
        <entry><literal>' 0.1'</literal></entry>
       </row>
       <row>
        <entry>to_char(12,'9990999.9')</entry>
        <entry><literal>'    0012.0'</literal></entry>
       </row>
       <row>
        <entry>to_char(12,'FM9990999.9')</entry>
        <entry><literal>'0012'</literal></entry>
       </row>
       <row>
        <entry>to_char(485,'999')</entry>
        <entry><literal>' 485'</literal></entry>
       </row>
       <row>
        <entry>to_char(-485,'999')</entry>
        <entry><literal>'-485'</literal></entry>
       </row>
       <row>
        <entry>to_char(485,'9 9 9')</entry>
        <entry><literal>' 4 8 5'</literal></entry>
       </row>
       <row>
        <entry>to_char(1485,'9,999')</entry>
        <entry><literal>' 1,485'</literal></entry>
       </row>
       <row>
        <entry>to_char(1485,'9G999')</entry>
        <entry><literal>' 1 485'</literal></entry>
       </row>
       <row>
        <entry>to_char(148.5,'999.999')</entry>
        <entry><literal>' 148.500'</literal></entry>
       </row>
       <row>
        <entry>to_char(148.5,'999D999')</entry>
        <entry><literal>' 148,500'</literal></entry>	 
       </row>
       <row>
        <entry>to_char(3148.5,'9G999D999')</entry>
        <entry><literal>' 3 148,500'</literal></entry>
       </row>
       <row>
        <entry>to_char(-485,'999S')</entry>
        <entry><literal>'485-'</literal></entry>
       </row>
       <row>		
        <entry>to_char(-485,'999MI')</entry>
        <entry><literal>'485-'</literal></entry>	
       </row>
       <row>
        <entry>to_char(485,'999MI')</entry>
        <entry><literal>'485'</literal></entry>		
       </row>
       <row>
        <entry>to_char(485,'PL999')</entry>
        <entry><literal>'+485'</literal></entry>	
       </row>
       <row>		
        <entry>to_char(485,'SG999')</entry>
        <entry><literal>'+485'</literal></entry>	
       </row>
       <row>
        <entry>to_char(-485,'SG999')</entry>
        <entry><literal>'-485'</literal></entry>	
       </row>
       <row>
        <entry>to_char(-485,'9SG99')</entry>
        <entry><literal>'4-85'</literal></entry>	
       </row>
       <row>
        <entry>to_char(-485,'999PR')</entry>
        <entry><literal>'&lt;485&gt;'</literal></entry>		
       </row>
       <row>
        <entry>to_char(485,'L999')</entry>
        <entry><literal>'DM 485</literal></entry>	 
       </row>
       <row>
        <entry>to_char(485,'RN')</entry>		
        <entry><literal>'        CDLXXXV'</literal></entry>
       </row>
       <row>
        <entry>to_char(485,'FMRN')</entry>	
        <entry><literal>'CDLXXXV'</literal></entry>
       </row>
       <row>
        <entry>to_char(5.2,'FMRN')</entry>
        <entry><literal>V</literal></entry>		
       </row>
       <row>
        <entry>to_char(482,'999th')</entry>
        <entry><literal>' 482nd'</literal></entry>				
       </row>
       <row>
        <entry>to_char(485, '"Good number:"999')</entry>
        <entry><literal>'Good number: 485'</literal></entry>
       </row>
       <row>
        <entry>to_char(485.8,'"Pre:"999" Post:" .999')</entry>
        <entry><literal>'Pre: 485 Post: .800'</literal></entry>
       </row>
       <row>
        <entry>to_char(12,'99V999')</entry>		
        <entry><literal>' 12000'</literal></entry>
       </row>
       <row>
        <entry>to_char(12.4,'99V999')</entry>
        <entry><literal>' 12400'</literal></entry>
       </row>
       <row>		
        <entry>to_char(12.45, '99V9')</entry>
        <entry><literal>' 125'</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>
  </sect1>


  <sect1 id="functions-datetime">
   <title>Date/Time Functions</title>

   <para>
    <xref linkend="functions-datetime-table"> shows the available
    functions for date/time value processing.  The basic arithmetic
    operators (<literal>+</literal>, <literal>*</literal>, etc.) are
    also available.  For formatting functions, refer to <xref
    linkend="functions-formatting">.  You should be familiar with the
    background information on date/time data types (see <xref
    linkend="datatype-datetime">).
   </para>

    <table id="functions-datetime-table">
     <title>Date/Time Functions</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Name</entry>
	<entry>Return Type</entry>
	<entry>Description</entry>
	<entry>Example</entry>
	<entry>Result</entry>
       </row>
      </thead>

      <tbody>
       <row>
	<entry>age(timestamp)</entry>
	<entry>interval</entry>
	<entry>Subtract from today</entry>
	<entry>age(timestamp '1957-06-13')</entry>
	<entry>43 years 8 mons 3 days</entry>
       </row>

       <row>
	<entry>age(timestamp, timestamp)</entry>
	<entry>interval</entry>
	<entry>Subtract arguments</entry>
	<entry>age('2001-04-10', timestamp '1957-06-13')</entry>
	<entry>43 years 9 mons 27 days</entry>
       </row>

       <row>
	<entry>current_date</entry>
	<entry>date</entry>
	<entry>Today's date; see <link
	 linkend="functions-datetime-current">below</link>
	</entry>
	<entry></entry>
	<entry></entry>
       </row>

       <row>
	<entry>current_time</entry>
	<entry>time</entry>
	<entry>Time of day; see <link
	 linkend="functions-datetime-current">below</link>
	</entry>
	<entry></entry>
	<entry></entry>
       </row>

       <row>
	<entry>current_timestamp</entry>
	<entry>timestamp</entry>
	<entry>date and time; see also <link
	 linkend="functions-datetime-current">below</link>
	</entry>
	<entry></entry>
	<entry></entry>
       </row>

       <row>
	<entry>date_part(text, timestamp)</entry>
	<entry>double precision</entry>
	<entry>Get subfield (equivalent to
	 <function>extract</function>); see also <link
	 linkend="functions-datetime-datepart">below</link>
        </entry>
	<entry>date_part('hour', timestamp '2001-02-16 20:38:40')</entry>
	<entry>20</entry>
       </row>

       <row>
	<entry>date_part(text, interval)</entry>
	<entry>double precision</entry>
	<entry>Get subfield (equivalent to
	 <function>extract</function>); see also <link
	 linkend="functions-datetime-datepart">below</link>
        </entry>
	<entry>date_part('month', interval '2 years 3 months')</entry>
	<entry>3</entry>
       </row>

       <row>
	<entry>date_trunc(text, timestamp)</entry>
	<entry>timestamp</entry>
	<entry>Truncate to specified precision; see also <link
         linkend="functions-datetime-trunc">below</link>
        </entry>
	<entry>date_trunc('hour', timestamp '2001-02-16 20:38:40')</entry>
	<entry>2001-02-16 20:00:00+00</entry>
       </row>

       <row>
	<entry>extract(<parameter>field</parameter> from timestamp)</entry>
	<entry>double precision</entry>
	<entry>Get subfield; see also <link
         linkend="functions-datetime-extract">below</link>
        </entry>
	<entry>extract(hour from timestamp '2001-02-16 20:38:40')</entry>
	<entry>20</entry>
       </row>

       <row>
	<entry>extract(<parameter>field</parameter> from interval)</entry>
	<entry>double precision</entry>
	<entry>Get subfield; see also <link
         linkend="functions-datetime-extract">below</link>
        </entry>
	<entry>extract(month from interval '2 years 3 months')</entry>
	<entry>3</entry>
       </row>

       <row>
	<entry>isfinite(timestamp)</entry>
	<entry>boolean</entry>
	<entry>Test for finite time stamp (neither invalid nor infinity)</entry>
	<entry>isfinite(timestamp '2001-02-16 21:28:30')</entry>
	<entry>true</entry>
       </row>

       <row>
	<entry>isfinite(interval)</entry>
	<entry>boolean</entry>
	<entry>Test for finite interval</entry>
	<entry>isfinite(interval '4 hours')</entry>
	<entry>true</entry>
       </row>

       <row>
	<entry>now()</entry>
	<entry>timestamp</entry>
	<entry>Current date and time (equivalent to
	 <function>current_timestamp</function>); see also <link
	 linkend="functions-datetime-current">below</link>
	</entry>
	<entry></entry>
	<entry></entry>
       </row>

       <row>
	<entry>timeofday()</entry>
	<entry>text</entry>
	<entry>High-precision date and time; see also <link
	 linkend="functions-datetime-current">below</link>
	</entry>
	<entry>timeofday()</entry>
	<entry>Wed Feb 21 17:01:13.000126 2001 EST</entry>
       </row>

       <row>
	<entry>timestamp(date)</entry>
	<entry>timestamp</entry>
	<entry>Date to timestamp</entry>
	<entry>timestamp(date '2000-12-25')</entry>
	<entry>2000-12-25 00:00:00</entry>
       </row>

       <row>
	<entry>timestamp(date, time)</entry>
	<entry>timestamp</entry>
	<entry>Date and time to a timestamp</entry>
	<entry>timestamp(date '1998-02-24',time '23:07')</entry>
	<entry>1998-02-24 23:07:00</entry>
       </row>
      </tbody>
     </tgroup>
    </table>


  <sect2 id="functions-datetime-extract">
   <title><function>EXTRACT</function>, <function>date_part</function></title>

<synopsis>
EXTRACT (<replaceable>field</replaceable> FROM <replaceable>source</replaceable>)
</synopsis>

   <para>
    The <function>extract</function> function retrieves sub-fields
    from date/time values, such as year or hour.
    <replaceable>source</replaceable> is a value expression that
    evaluates to type <type>timestamp</type> or <type>interval</type>.
    (Expressions of type <type>date</type> or <type>time</type> will
    be cast to <type>timestamp</type> and can therefore be used as
    well.)  <replaceable>field</replaceable> is an identifier or
    string that selects what field to extract from the source value.
    The <function>extract</function> function returns values of type
    <type>double precision</type>.
    The following are valid values:

    <!-- alphabetical -->
    <variablelist>
     <varlistentry>
      <term>century</term>
      <listitem>
       <para>
        The year field divided by 100
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
</screen>
       </informalexample>

       <para>
        Note that the result for the century field is simply the year field
        divided by 100, and not the conventional definition which puts most
        years in the 1900's in the twentieth century.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>day</term>
      <listitem>
       <para>
        The day (of the month) field (1 - 31)
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>decade</term>
      <listitem>
       <para>
        The year field divided by 10
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>200</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>dow</term>
      <listitem>
       <para>
        The day of the week (0 - 6; Sunday is 0) (for
        <type>timestamp</type> values only)
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>5</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>doy</term>
      <listitem>
       <para>
        The day of the year (1 - 365/366) (for <type>timestamp</type> values only)
       </para>
       <informalexample>
<screen>
SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>47</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>epoch</term>
      <listitem>
       <para>
        For <type>date</type> and <type>timestamp</type> values, the
        number of seconds since 1970-01-01 00:00:00 (Result may be
        negative.); for <type>interval</type> values, the total number
        of seconds in the interval
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>982352320</computeroutput>

SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours');
<lineannotation>Result: </lineannotation><computeroutput>442800</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>hour</term>
      <listitem>
       <para>
        The hour field (0 - 23)
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>20</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>microseconds</term>
      <listitem>
       <para>
        The seconds field, including fractional parts, multiplied by 1
        000 000.  Note that this includes full seconds.
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28500000</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>millennium</term>
      <listitem>
       <para>
        The year field divided by 1000
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>
</screen>
       </informalexample>

       <para>
        Note that the result for the millennium field is simply the year field
        divided by 1000, and not the conventional definition which puts
        years in the 1900's in the second millennium.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>milliseconds</term>
      <listitem>
       <para>
        The seconds field, including fractional parts, multiplied by
        1000.  Note that this includes full seconds.
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28500</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>minute</term>
      <listitem>
       <para>
        The minutes field (0 - 59)
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>38</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>month</term>
      <listitem>
       <para>
        For <type>timestamp</type> values, the number of the month
        within the year (1 - 12) ; for <type>interval</type> values
        the number of months, modulo 12 (0 - 11)
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2</computeroutput>

SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months');
<lineannotation>Result: </lineannotation><computeroutput>3</computeroutput>

SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>quarter</term>
      <listitem>
       <para>
        The quarter of the year (1 - 4) that the day is in (for
        <type>timestamp</type> values only)
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>1</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>second</term>
      <listitem>
       <para>
        The seconds field, including fractional parts (0 -
        59<footnote><simpara>60 if leap seconds are
        implemented by the operating system</simpara></footnote>)
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>40</computeroutput>

SELECT EXTRACT(SECOND FROM TIME '17:12:28.5');
<lineannotation>Result: </lineannotation><computeroutput>28.5</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>week</term>
      <listitem>
       <para>
        From a <type>timestamp</type> value, calculate the number of
        the week of the year that the day is in.  By definition
        (<acronym>ISO</acronym> 8601), the first week of a year
        contains January 4 of that year.  (The <acronym>ISO</acronym>
        week starts on Monday.)  In other words, the first Thursday of
        a year is in week 1 of that year.
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>7</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>year</term>
      <listitem>
       <para>
        The year field
       </para>

       <informalexample>
<screen>
SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001</computeroutput>
</screen>
       </informalexample>
      </listitem>
     </varlistentry>

<!--
tz
tz_hour
tz_minute
-->

    </variablelist>

   </para>

   <para>
    The <function>extract</function> function is primarily intended
    for computational processing.  For formatting date/time values for
    display, see <xref linkend="functions-formatting">.
   </para>

   <anchor id="functions-datetime-datepart">
   <para>
    The <function>date_part</function> function is modeled on the traditional
    <productname>Ingres</productname> equivalent to the
    <acronym>SQL</acronym>-function <function>extract</function>:
<synopsis>
date_part('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
</synopsis>
    Note that here the <replaceable>field</replaceable> value needs to
    be a string.  The valid field values for
    <function>date_part</function> are the same as for
    <function>extract</function>.
   </para>

   <informalexample>
<screen>
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>16</computeroutput>

SELECT date_part('hour', INTERVAL '4 hours 3 minutes')
<lineannotation>Result: </lineannotation><computeroutput>4</computeroutput>
</screen>
   </informalexample>

  </sect2>

  <sect2 id="functions-datetime-trunc">
   <title><function>date_trunc</function></title>

   <para>
    The function <function>date_trunc</function> is conceptually
    similar to the <function>trunc</function> function for numbers.
   </para>

   <para>
<synopsis>
date_trunc('<replaceable>field</replaceable>', <replaceable>source</replaceable>)
</synopsis>
    <replaceable>source</replaceable> is a value expression of type
    <type>timestamp</type> (values of type <type>date</type> and
    <type>time</type> are cast automatically).
    <replaceable>field</replaceable> selects to which precision to
    truncate the time stamp value.  The return value is of type
    <type>timestamp</type> with all fields that are less than the
    selected one set to zero (or one, for day and month).
   </para>

   <para>
    Valid values for <replaceable>field</replaceable> are:
    <simplelist>
     <member>microseconds</member>
     <member>milliseconds</member>
     <member>second</member>
     <member>minute</member>
     <member>hour</member>
     <member>day</member>
     <member>month</member>
     <member>year</member>
     <member>decade</member>
     <member>century</member>
     <member>millennium</member>
    </simplelist>
   </para>

   <informalexample>
    <para>
<screen>
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-02-16 20:00:00+00</computeroutput>

SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40');
<lineannotation>Result: </lineannotation><computeroutput>2001-01-01 00:00:00+00</computeroutput>
</screen>
    </para>
   </informalexample>
  </sect2>

  <sect2 id="functions-datetime-current">
   <title>Current Date/Time</title>

   <indexterm>
    <primary>date</primary>
    <secondary>current</secondary>
   </indexterm>

   <indexterm>
    <primary>time</primary>
    <secondary>current</secondary>
   </indexterm>

   <para>
    The following functions are available to obtain the current date and/or
    time:
<synopsis>
CURRENT_TIME
CURRENT_DATE
CURRENT_TIMESTAMP
</synopsis>
    Note that because of the requirements of the
    <acronym>SQL</acronym> standard, these functions must not be
    called with trailing parentheses.
   </para>

   <informalexample>
<screen>
SELECT CURRENT_TIME;
<computeroutput>19:07:32</computeroutput>

SELECT CURRENT_DATE;
<computeroutput>2001-02-17</computeroutput>

SELECT CURRENT_TIMESTAMP;
<computeroutput>2001-02-17 19:07:32-05</computeroutput>
</screen>
   </informalexample>

   <para>
    The function <function>now()</function> is the traditional
    <productname>Postgres</productname> equivalent to
    <function>CURRENT_TIMESTAMP</function>.
   </para>

   <para>
    There is also <function>timeofday()</function>, which returns current
    time to higher precision than the <function>CURRENT_TIMESTAMP</function>
    family does:
   </para>

   <informalexample>
<screen>
SELECT timeofday();
 Sat Feb 17 19:07:32.000126 2001 EST
</screen>
   </informalexample>

   <para>
    <function>timeofday()</function> uses the operating system call
    <function>gettimeofday(2)</function>, which may have resolution as
    good as microseconds (depending on your platform); the other functions
    rely on <function>time(2)</function> which is restricted to one-second
    resolution.  For historical reasons, <function>timeofday()</function>
    returns its result as a text string rather than a timestamp value.
   </para>

   <para>
    It is quite important to realize that
    <function>CURRENT_TIMESTAMP</function> and related functions all return
    the time as of the start of the current transaction; their values do not
    increment while a transaction is running.  But
    <function>timeofday()</function> returns the actual current time.
   </para>

   <para>
    All the date/time datatypes also accept the special literal value
    <literal>now</> to specify the current date and time.  Thus,
    the following three all return the same result:
<programlisting>
SELECT CURRENT_TIMESTAMP;
SELECT now();
SELECT TIMESTAMP 'now';
</programlisting>
    <note>
     <para>
      You do not want to use the third form when specifying a DEFAULT
      value while creating a table.  The system will convert <literal>now</>
      to a timestamp as soon as the constant is parsed, so that when
      the default value is needed,
      the time of the table creation would be used!  The first two
      forms will not be evaluated until the default value is used,
      because they are function calls.  Thus they will give the desired
      behavior of defaulting to the time of row insertion.
     </para>
    </note>
   </para>
  </sect2>
 </sect1>

  
 <sect1 id="functions-geometry">
   <title>Geometric Functions and Operators</title>

   <para>
    The geometric types point, box, lseg, line, path, polygon, and
    circle have a large set of native support functions and operators.
   </para>

   <table>
     <TITLE>Geometric Operators</TITLE>
     <TGROUP COLS="3">
      <THEAD>
       <ROW>
	<ENTRY>Operator</ENTRY>
	<ENTRY>Description</ENTRY>
	<ENTRY>Usage</ENTRY>
       </ROW>
      </THEAD>
      <TBODY>
       <ROW>
	<ENTRY> + </ENTRY>
	<ENTRY>Translation</ENTRY>
	<ENTRY>box '((0,0),(1,1))' + point '(2.0,0)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> - </ENTRY>
	<ENTRY>Translation</ENTRY>
	<ENTRY>box '((0,0),(1,1))' - point '(2.0,0)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> * </ENTRY>
	<ENTRY>Scaling/rotation</ENTRY>
	<ENTRY>box '((0,0),(1,1))' * point '(2.0,0)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> / </ENTRY>
	<ENTRY>Scaling/rotation</ENTRY>
	<ENTRY>box '((0,0),(2,2))' / point '(2.0,0)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> # </ENTRY>
	<ENTRY>Intersection</ENTRY>
	<ENTRY>'((1,-1),(-1,1))' # '((1,1),(-1,-1))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> # </ENTRY>
	<ENTRY>Number of points in polygon</ENTRY>
	<ENTRY># '((1,0),(0,1),(-1,0))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> ## </ENTRY>
	<ENTRY>Point of closest proximity</ENTRY>
	<ENTRY>point '(0,0)' ## lseg '((2,0),(0,2))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &amp;&amp; </ENTRY>
	<ENTRY>Overlaps?</ENTRY>
	<ENTRY>box '((0,0),(1,1))' &amp;&amp; box '((0,0),(2,2))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &amp;&lt; </ENTRY>
	<ENTRY>Overlaps to left?</ENTRY>
	<ENTRY>box '((0,0),(1,1))' &amp;&lt; box '((0,0),(2,2))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &amp;&gt; </ENTRY>
	<ENTRY>Overlaps to right?</ENTRY>
	<ENTRY>box '((0,0),(3,3))' &amp;&gt; box '((0,0),(2,2))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &lt;-&gt; </ENTRY>
	<ENTRY>Distance between</ENTRY>
	<ENTRY>circle '((0,0),1)' &lt;-&gt; circle '((5,0),1)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &lt;&lt; </ENTRY>
	<ENTRY>Left of?</ENTRY>
	<ENTRY>circle '((0,0),1)' &lt;&lt; circle '((5,0),1)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &lt;^ </ENTRY>
	<ENTRY>Is below?</ENTRY>
	<ENTRY>circle '((0,0),1)' &lt;^ circle '((0,5),1)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &gt;&gt; </ENTRY>
	<ENTRY>Is right of?</ENTRY>
	<ENTRY>circle '((5,0),1)' &gt;&gt; circle '((0,0),1)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &gt;^ </ENTRY>
	<ENTRY>Is above?</ENTRY>
	<ENTRY>circle '((0,5),1)' >^ circle '((0,0),1)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> ?# </ENTRY>
	<ENTRY>Intersects or overlaps</ENTRY>
	<ENTRY>lseg '((-1,0),(1,0))' ?# box '((-2,-2),(2,2))';</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> ?- </ENTRY>
	<ENTRY>Is horizontal?</ENTRY>
	<ENTRY>point '(1,0)' ?- point '(0,0)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> ?-| </ENTRY>
	<ENTRY>Is perpendicular?</ENTRY>
	<ENTRY>lseg '((0,0),(0,1))' ?-| lseg '((0,0),(1,0))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> @-@  </ENTRY>
	<ENTRY>Length or circumference</ENTRY>
	<ENTRY>@-@ path '((0,0),(1,0))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> ?| </ENTRY>
	<ENTRY>Is vertical?</ENTRY>
	<ENTRY>point '(0,1)' ?| point '(0,0)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> ?|| </ENTRY>
	<ENTRY>Is parallel?</ENTRY>
	<ENTRY>lseg '((-1,0),(1,0))' ?|| lseg '((-1,2),(1,2))'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> @ </ENTRY>
	<ENTRY>Contained or on</ENTRY>
	<ENTRY>point '(1,1)' @ circle '((0,0),2)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> @@ </ENTRY>
	<ENTRY>Center of</ENTRY>
	<ENTRY>@@ circle '((0,0),10)'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> ~= </ENTRY>
	<ENTRY>Same as</ENTRY>
	<ENTRY>polygon '((0,0),(1,1))' ~= polygon '((1,1),(0,0))'</ENTRY>
       </ROW>
      </TBODY>
     </TGROUP>
   </TABLE>

   <table>
     <title>Geometric Functions</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Function</entry>
	<entry>Returns</entry>
	<entry>Description</entry>
	<entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>area(object)</entry>
	<entry>double precision</entry>
	<entry>area of item</entry>
	<entry>area(box '((0,0),(1,1))')</entry>
       </row>
       <row>
	<entry>box(box, box)</entry>
	<entry>box</entry>
	<entry>intersection box</entry>
	<entry>box(box '((0,0),(1,1))',box '((0.5,0.5),(2,2))')</entry>
       </row>
       <row>
	<entry>center(object)</entry>
	<entry>point</entry>
	<entry>center of item</entry>
	<entry>center(box '((0,0),(1,2))')</entry>
       </row>
       <row>
	<entry>diameter(circle)</entry>
	<entry>double precision</entry>
	<entry>diameter of circle</entry>
	<entry>diameter(circle '((0,0),2.0)')</entry>
       </row>
       <row>
	<entry>height(box)</entry>
	<entry>double precision</entry>
	<entry>vertical size of box</entry>
	<entry>height(box '((0,0),(1,1))')</entry>
       </row>
       <row>
	<entry>isclosed(path)</entry>
	<entry>boolean</entry>
	<entry>a closed path?</entry>
	<entry>isclosed(path '((0,0),(1,1),(2,0))')</entry>
       </row>
       <row>
	<entry>isopen(path)</entry>
	<entry>boolean</entry>
	<entry>an open path?</entry>
	<entry>isopen(path '[(0,0),(1,1),(2,0)]')</entry>
       </row>
       <row>
	<entry>length(object)</entry>
	<entry>double precision</entry>
	<entry>length of item</entry>
	<entry>length(path '((-1,0),(1,0))')</entry>
       </row>
       <row>
	<entry>pclose(path)</entry>
	<entry>path</entry>
	<entry>convert path to closed</entry>
	<entry>popen(path '[(0,0),(1,1),(2,0)]')</entry>
       </row>
<!--
Not defined by this name. Implements the intersection operator '#'
       <row>
	<entry>point(lseg,lseg)</entry>
	<entry>point</entry>
	<entry>intersection</entry>
	<entry>point(lseg '((-1,0),(1,0))',lseg '((-2,-2),(2,2))')</entry>
       </row>
-->
       <row>
	<entry>npoint(path)</entry>
	<entry>int4</entry>
	<entry>number of points</entry>
	<entry>npoints(path '[(0,0),(1,1),(2,0)]')</entry>
       </row>
       <row>
	<entry>popen(path)</entry>
	<entry>path</entry>
	<entry>convert path to open path</entry>
	<entry>popen(path '((0,0),(1,1),(2,0))')</entry>
       </row>
       <row>
	<entry>radius(circle)</entry>
	<entry>double precision</entry>
	<entry>radius of circle</entry>
	<entry>radius(circle '((0,0),2.0)')</entry>
       </row>
       <row>
	<entry>width(box)</entry>
	<entry>double precision</entry>
	<entry>horizontal size</entry>
	<entry>width(box '((0,0),(1,1))')</entry>
       </row>
      </tbody>
     </tgroup>
   </table>


   <table>
     <title>Geometric Type Conversion Functions</title>
     <tgroup cols="4">
      <thead>
       <row>
	<entry>Function</entry>
	<entry>Returns</entry>
	<entry>Description</entry>
	<entry>Example</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>box(circle)</entry>
	<entry>box</entry>
	<entry>circle to box</entry>
	<entry>box(circle '((0,0),2.0)')</entry>
       </row>
       <row>
	<entry>box(point, point)</entry>
	<entry>box</entry>
	<entry>points to box</entry>
	<entry>box(point '(0,0)', point '(1,1)')</entry>
       </row>
       <row>
	<entry>box(polygon)</entry>
	<entry>box</entry>
	<entry>polygon to box</entry>
	<entry>box(polygon '((0,0),(1,1),(2,0))')</entry>
       </row>
       <row>
	<entry>circle(box)</entry>
	<entry>circle</entry>
	<entry>to circle</entry>
	<entry>circle(box '((0,0),(1,1))')</entry>
       </row>
       <row>
	<entry>circle(point, double precision)</entry>
	<entry>circle</entry>
	<entry>point to circle</entry>
	<entry>circle(point '(0,0)', 2.0)</entry>
       </row>
       <row>
	<entry>lseg(box)</entry>
	<entry>lseg</entry>
	<entry>box diagonal to lseg</entry>
	<entry>lseg(box '((-1,0),(1,0))')</entry>
       </row>
       <row>
	<entry>lseg(point, point)</entry>
	<entry>lseg</entry>
	<entry>points to lseg</entry>
	<entry>lseg(point '(-1,0)', point '(1,0)')</entry>
       </row>
       <row>
	<entry>path(polygon)</entry>
	<entry>point</entry>
	<entry>polygon to path</entry>
	<entry>path(polygon '((0,0),(1,1),(2,0))')</entry>
       </row>
       <row>
	<entry>point(circle)</entry>
	<entry>point</entry>
	<entry>center</entry>
	<entry>point(circle '((0,0),2.0)')</entry>
       </row>
       <row>
	<entry>point(lseg, lseg)</entry>
	<entry>point</entry>
	<entry>intersection</entry>
	<entry>point(lseg '((-1,0),(1,0))', lseg '((-2,-2),(2,2))')</entry>
       </row>
       <row>
	<entry>point(polygon)</entry>
	<entry>point</entry>
	<entry>center</entry>
	<entry>point(polygon '((0,0),(1,1),(2,0))')</entry>
       </row>
       <row>
	<entry>polygon(box)</entry>
	<entry>polygon</entry>
	<entry>12 point polygon</entry>
	<entry>polygon(box '((0,0),(1,1))')</entry>
       </row>
       <row>
	<entry>polygon(circle)</entry>
	<entry>polygon</entry>
	<entry>12-point polygon</entry>
	<entry>polygon(circle '((0,0),2.0)')</entry>
       </row>
       <row>
	<entry>polygon(<replaceable class="parameter">npts</replaceable>, circle)</entry>
	<entry>polygon</entry>
	<entry><replaceable class="parameter">npts</replaceable> polygon</entry>
	<entry>polygon(12, circle '((0,0),2.0)')</entry>
       </row>
       <row>
	<entry>polygon(path)</entry>
	<entry>polygon</entry>
	<entry>path to polygon</entry>
	<entry>polygon(path '((0,0),(1,1),(2,0))')</entry>
       </row>
      </tbody>
     </tgroup>
   </table>

  </sect1>


  <sect1 id="functions-net">
   <title>Network Address Type Functions</title>


    <table tocentry="1" id="cidr-inet-operators-table">
     <title><type>cidr</> and <type>inet</> Operators</title>
     <TGROUP COLS="3">
      <THEAD>
       <ROW>
	<ENTRY>Operator</ENTRY>
	<ENTRY>Description</ENTRY>
	<ENTRY>Usage</ENTRY>
       </ROW>
      </THEAD>
      <TBODY>
       <ROW>
	<ENTRY> &lt; </ENTRY>
	<ENTRY>Less than</ENTRY>
	<ENTRY>inet '192.168.1.5' &lt; inet '192.168.1.6'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &lt;= </ENTRY>
	<ENTRY>Less than or equal</ENTRY>
	<ENTRY>inet '192.168.1.5' &lt;= inet '192.168.1.5'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> = </ENTRY>
	<ENTRY>Equals</ENTRY>
	<ENTRY>inet '192.168.1.5' = inet '192.168.1.5'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &gt;= </ENTRY>
	<ENTRY>Greater or equal</ENTRY>
	<ENTRY>inet '192.168.1.5' &gt;= inet '192.168.1.5'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &gt; </ENTRY>
	<ENTRY>Greater</ENTRY>
	<ENTRY>inet '192.168.1.5' &gt; inet '192.168.1.4'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &lt;&gt; </ENTRY>
	<ENTRY>Not equal</ENTRY>
	<ENTRY>inet '192.168.1.5' &lt;&gt; inet '192.168.1.4'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &lt;&lt; </ENTRY>
	<ENTRY>is contained within</ENTRY>
	<ENTRY>inet '192.168.1.5' &lt;&lt; inet '192.168.1/24'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &lt;&lt;= </ENTRY>
	<ENTRY>is contained within or equals</ENTRY>
	<ENTRY>inet '192.168.1/24' &lt;&lt;= inet '192.168.1/24'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &gt;&gt; </ENTRY>
	<ENTRY>contains</ENTRY>
	<ENTRY>inet'192.168.1/24' &gt;&gt; inet '192.168.1.5'</ENTRY>
       </ROW>
       <ROW>
	<ENTRY> &gt;&gt;= </ENTRY>
	<ENTRY>contains or equals</ENTRY>
	<ENTRY>inet '192.168.1/24' &gt;&gt;= inet '192.168.1/24'</ENTRY>
       </ROW>
      </TBODY>
     </TGROUP>
    </TABLE>

    <para>
     All of the operators for <type>inet</type> can be applied to
     <type>cidr</type> values as well.  The operators
     <literal>&lt;&lt;</>, <literal>&lt;&lt;=</>,
     <literal>&gt;&gt;</>, <literal>&gt;&gt;=</>
     test for subnet inclusion: they consider only the network parts
     of the two addresses, ignoring any host part, and determine whether
     one network part is identical to or a subnet of the other.
    </para>


    <table tocentry="1" id="cidr-inet-functions">
     <title><type>cidr</> and <type>inet</> Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
	<entry>Function</entry>
	<entry>Returns</entry>
	<entry>Description</entry>
	<entry>Example</entry>
	<entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>broadcast(inet)</entry>
	<entry>inet</entry>
	<entry>broadcast address for network</entry>
	<entry>broadcast('192.168.1.5/24')</entry>
	<entry>192.168.1.255/24</entry>
       </row>
       <row>
	<entry>host(inet)</entry>
	<entry>text</entry>
	<entry>extract IP address as text</entry>
	<entry>host('192.168.1.5/24')</entry>
	<entry>192.168.1.5</entry>
       </row>
       <row>
	<entry>masklen(inet)</entry>
	<entry>integer</entry>
	<entry>extract netmask length</entry>
	<entry>masklen('192.168.1.5/24')</entry>
	<entry>24</entry>
       </row>
       <row>
	<entry>set_masklen(inet,integer)</entry>
	<entry>inet</entry>
	<entry>set netmask length for inet value</entry>
	<entry>set_masklen('192.168.1.5/24',16)</entry>
	<entry>192.168.1.5/16</entry>
       </row>
       <row>
	<entry>netmask(inet)</entry>
	<entry>inet</entry>
	<entry>construct netmask for network</entry>
	<entry>netmask('192.168.1.5/24')</entry>
	<entry>255.255.255.0</entry>
       </row>
       <row>
	<entry>network(inet)</entry>
	<entry>cidr</entry>
	<entry>extract network part of address</entry>
	<entry>network('192.168.1.5/24')</entry>
	<entry>192.168.1.0/24</entry>
       </row>
       <row>
	<entry>text(inet)</entry>
	<entry>text</entry>
	<entry>extract IP address and masklen as text</entry>
	<entry>text(inet '192.168.1.5')</entry>
	<entry>192.168.1.5/32</entry>
       </row>
       <row>
	<entry>abbrev(inet)</entry>
	<entry>text</entry>
	<entry>extract abbreviated display as text</entry>
	<entry>abbrev(cidr '10.1.0.0/16')</entry>
	<entry>10.1/16</entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    All of the functions for <type>inet</type> can be applied to
    <type>cidr</type> values as well.  The <function>host</>(),
    <function>text</>(), and <function>abbrev</>() functions are primarily
    intended to offer alternative display formats. You can cast a text
    field to inet using normal casting syntax: inet(fieldname) or 
    fieldname::inet.
   </para>

   <para>
    <table tocentry="1" id="macaddr-functions">
     <title><type>macaddr</> Functions</title>
     <tgroup cols="5">
      <thead>
       <row>
	<entry>Function</entry>
	<entry>Returns</entry>
	<entry>Description</entry>
	<entry>Example</entry>
	<entry>Result</entry>
       </row>
      </thead>
      <tbody>
       <row>
	<entry>trunc(macaddr)</entry>
	<entry>macaddr</entry>
	<entry>set last 3 bytes to zero</entry>
	<entry>trunc(macaddr '12:34:56:78:90:ab')</entry>
	<entry>12:34:56:00:00:00</entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </para>

   <para>
    The function <function>trunc</>(<type>macaddr</>) returns a MAC
    address with the last 3 bytes set to 0.  This can be used to
    associate the remaining prefix with a manufacturer.  The directory
    <filename>contrib/mac</> in the source distribution contains some
    utilities to create and maintain such an association table.
   </para>

   <para>
    The <type>macaddr</> type also supports the standard relational
    operators (<literal>&gt;</>, <literal>&lt;=</>, etc.) for
    lexicographical ordering.
   </para>

  </sect1>


 <sect1 id="functions-conditional">
  <title>Conditional Expressions</title>

  <indexterm>
   <primary>case</primary>
  </indexterm>

  <indexterm>
   <primary>conditionals</primary>
  </indexterm>

  <para>
   This section describes the <acronym>SQL</acronym>-compliant conditional expressions
   available in <productname>Postgres</productname>.
  </para>

  <tip>
   <para>
    If your needs go beyond the capabilities of these conditional
    expressions you might want to consider writing a stored procedure
    in a more expressive programming language.
   </para>
  </tip>

  <bridgehead renderas="sect2">CASE</bridgehead>

<synopsis>
CASE WHEN <replaceable>condition</replaceable> THEN <replaceable>result</replaceable>
     <optional>WHEN ...</optional>
     <optional>ELSE <replaceable>result</replaceable></optional>
END
</synopsis>

  <para>
   The <acronym>SQL</acronym> <token>CASE</token> expression is a
   generic conditional expression, similar to if/else statements in
   other languages.  <token>CASE</token> clauses can be used wherever
   an expression is valid.  <replaceable>condition</replaceable> is an
   expression that returns a <type>boolean</type> result.  If the result is true
   then the value of the <token>CASE</token> expression is
   <replaceable>result</replaceable>.  If the result is false any
   subsequent <token>WHEN</token> clauses are searched in the same
   manner.  If no <token>WHEN</token>
   <replaceable>condition</replaceable> is true then the value of the
   case expression is the <replaceable>result</replaceable> in the
   <token>ELSE</token> clause.  If the <token>ELSE</token> clause is
   omitted and no condition matches, the result is NULL.
  </para>

  <informalexample>
   <para>
    An example:
<screen>
<prompt>=&gt;</prompt> <userinput>SELECT * FROM test;</userinput>
<computeroutput>
 a
---
 1
 2
 3
</computeroutput>

<prompt>=&gt;</prompt> <userinput>SELECT a, CASE WHEN a=1 THEN 'one' WHEN a=2 THEN 'two' ELSE 'other' END FROM test;</userinput>
<computeroutput>
 a | case
---+-------
 1 | one
 2 | two
 3 | other
</computeroutput>
</screen>
   </para>
  </informalexample>

  <para>
   The data types of all the <replaceable>result</replaceable>
   expressions must be coercible to a single output type.
   See <xref linkend="typeconv-union-case"> for more detail.
  </para>

<synopsis>
CASE <replaceable>expression</replaceable>
    WHEN <replaceable>value</replaceable> THEN <replaceable>result</replaceable>
    <optional>WHEN ...</optional>
    <optional>ELSE <replaceable>result</replaceable></optional>
END
</synopsis>

  <para>
   This <quote>simple</quote> <token>CASE</token> expression is a
   specialized variant of the general form above.  The
   <replaceable>expression</replaceable> is computed and compared to
   all the <replaceable>value</replaceable>s in the
   <token>WHEN</token> clauses until one is found that is equal.  If
   no match is found, the <replaceable>result</replaceable> in the
   <token>ELSE</token> clause (or NULL) is returned.  This is similar
   to the <function>switch</function> statement in C.
  </para>

  <informalexample>
   <para>
    The example above can be written using the simple
    <token>CASE</token> syntax:
<screen>
<prompt>=&gt;</prompt> <userinput>SELECT a, CASE a WHEN 1 THEN 'one' WHEN 2 THEN 'two' ELSE 'other' END FROM test;</userinput>
<computeroutput>
 a | case
---+-------
 1 | one
 2 | two
 3 | other
</computeroutput>
</screen>
    </para>
   </informalexample>

   <bridgehead renderas="sect2">COALESCE</bridgehead>

<synopsis>
<function>COALESCE</function>(<replaceable>value</replaceable><optional>, ...</optional>)
</synopsis>

  <para>
   The <function>COALESCE</function> function returns the first of its
   arguments that is not NULL.  This is often useful to substitute a
   default value for NULL values when data is retrieved for display,
   for example:
<programlisting>
SELECT COALESCE(description, short_description, '(none)') ...
</programlisting>
  </para>

 <bridgehead renderas="sect2">NULLIF</bridgehead>

  <indexterm>
   <primary>nullif</primary>
  </indexterm>

<synopsis>
<function>NULLIF</function>(<replaceable>value1</replaceable>, <replaceable>value2</replaceable>)
</synopsis>

  <para>
   The <function>NULLIF</function> function returns NULL if and only
   if <replaceable>value1</replaceable> and
   <replaceable>value2</replaceable> are equal.  Otherwise it returns
   <replaceable>value1</replaceable>.  This can be used to perform the
   inverse operation of the <function>COALESCE</function> example
   given above:
<programlisting>
SELECT NULLIF(value, '(none)') ...
</programlisting>
  </para>

  <tip>
   <para>
    <function>COALESCE</function> and <function>NULLIF</function> are
    just shorthand for <token>CASE</token> expressions.  They are actually
    converted into <token>CASE</token> expressions at a very early stage
    of processing, and subsequent processing thinks it is dealing with
    <token>CASE</token>.  Thus an incorrect <function>COALESCE</function> or
    <function>NULLIF</function> usage may draw an error message that
    refers to <token>CASE</token>.
   </para>
  </tip>

 </sect1>


  <sect1 id="functions-misc">
   <title>Miscellaneous Functions</>

   <table>
    <title>Session Information Functions</>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</> <entry>Return Type</> <entry>Description</></row>
     </thead>

     <tbody>
      <row>
       <entry>current_user</>
       <entry>name</>
       <entry>user name of current execution context</>
      </row>
      <row>
       <entry>session_user</>
       <entry>name</>
       <entry>session user name</>
      </row>
      <row>
       <entry>user</>
       <entry>name</>
       <entry>equivalent to <function>current_user</></>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm zone="functions-misc">
    <primary>user</primary>
    <secondary>current</secondary>
   </indexterm>

   <para>
    The <function>session_user</> is the user that initiated a database
    connection and is fixed for the duration of that connection. The
    <function>current_user</> is the user identifier that is applicable
    for permission checking. Currently it is always equal to the session
    user, but in the future there might be <quote>setuid</> functions and
    other facilities to allow the current user to change temporarily.
    In Unix parlance, the session user is the <quote>real user</>
    and the current user is the <quote>effective user</>.
   </para>

   <para>
    Note that these functions have special syntactic status in <acronym>SQL</>;
    they must be called without trailing parentheses.
   </para>

   <note>
    <title>Deprecated</>
    <para>
     The function <function>getpgusername()</> is an obsolete equivalent
     of <function>current_user</>.
    </para>
   </note>

   <table>
    <title>Access Privilege Inquiry Functions</>
    <tgroup cols="3">
     <thead>
      <row><entry>Name</> <entry>Return Type</> <entry>Description</></row>
     </thead>

     <tbody>
      <row>
       <entry>has_table_privilege(<parameter>user</parameter>,
                                  <parameter>table</parameter>,
                                  <parameter>access</parameter>)
       </entry>
       <entry>boolean</>
       <entry>does user have access to table</>
      </row>
      <row>
       <entry>has_table_privilege(<parameter>table</parameter>,
                                  <parameter>access</parameter>)
       </entry>
       <entry>boolean</>
       <entry>does current user have access to table</>
      </row>
     </tbody>
    </tgroup>
   </table>

   <indexterm zone="functions-misc">
    <primary>has_table_privilege</primary>
   </indexterm>

   <para>
    <function>has_table_privilege</> determines whether a user
    can access a table in a particular way.  The user can be
    specified by name or by usesysid, or if the argument is omitted
    <function>current_user</> is assumed.  The table can be specified
    by name or by OID.  (Thus, there are actually six variants of
    <function>has_table_privilege</>, which can be distinguished by
    the number and types of their arguments.)  The desired access type
    is specified by a text string, which must evaluate to one of the
    values <literal>SELECT</>, <literal>INSERT</>, <literal>UPDATE</>,
    <literal>DELETE</>, <literal>RULE</>, <literal>REFERENCES</>, or
    <literal>TRIGGER</>.  (Case of the string is not significant, however.)
   </para>

  </sect1>


 <sect1 id="functions-aggregate">
  <title>Aggregate Functions</title>

  <note>
   <title>Author</title>
   <para>
    Written by Isaac Wilcox <email>isaac@azartmedia.com</email> on 2000-06-16
   </para>
  </note>

  <para>
   <firstterm>Aggregate functions</firstterm> compute a single result
   value from a set of input values.  The special syntax
   considerations for aggregate functions are explained in <xref
   linkend="syntax-aggregates">.  Consult the <citetitle>PostgreSQL
   Tutorial</citetitle> for additional introductory information.
  </para>

  <table tocentry="1">
   <title>Aggregate Functions</title>

   <tgroup cols="3">
    <thead>
     <row>
      <entry>Function</entry>
      <entry>Description</entry>
      <entry>Notes</entry>
     </row>
    </thead>

    <tbody>
     <row>
      <entry>AVG(<replaceable class="parameter">expression</replaceable>)</entry>
      <entry>the average (arithmetic mean) of all input values</entry>
      <entry>
       <indexterm>
        <primary>average</primary>
        <secondary>function</secondary>
       </indexterm>
       Finding the average value is available on the following data
       types: <type>smallint</type>, <type>integer</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, <type>numeric</type>, <type>interval</type>.
       The result is of type <type>numeric</type> for any integer type
       input, <type>double precision</type> for floating point input,
       otherwise the same as the input data type.
      </entry>
     </row>

     <row>
      <entry>COUNT(*)</entry>
      <entry>number of input values</entry>
      <entry>The return value is of type <type>bigint</type>.</entry>
     </row>

     <row>
      <entry>COUNT(<replaceable class="parameter">expression</replaceable>)</entry>
      <entry>
       Counts the input values for which the value of <replaceable
       class="parameter">expression</replaceable> is not NULL.
      </entry>
      <entry>The return value is of type <type>bigint</type>.</entry>
     </row>

     <row>
      <entry>MAX(<replaceable class="parameter">expression</replaceable>)</entry>
      <entry>the maximum value of <replaceable class="parameter">expression</replaceable> across all input values</entry>
      <entry>
       Available for all numeric, string, and date/time types.  The
       result has the same type as the input expression.
      </entry>
     </row>

     <row>
      <entry>MIN(<replaceable class="parameter">expression</replaceable>)</entry>
      <entry>the minimum value of <replaceable class="parameter">expression</replaceable> across all input values</entry>
      <entry>
       Available for all numeric, string, and date/time types.  The
       result has the same type as the input expression.
      </entry>
     </row>

     <row>
      <entry>STDDEV(<replaceable class="parameter">expression</replaceable>)</entry>
      <entry>the sample standard deviation of the input values</entry>
      <entry>
       <indexterm>
        <primary>standard deviation</primary>
       </indexterm>
       Finding the standard deviation is available on the following
       data types: <type>smallint</type>, <type>integer</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, <type>numeric</type>.  The result is of type
       <type>double precision</type> for floating point input,
       otherwise <type>numeric</type>.
      </entry>
     </row>

     <row>
      <entry>SUM(<replaceable class="parameter">expression</replaceable>)</entry>
      <entry>sum of <replaceable class="parameter">expression</replaceable> across all input values</entry>
      <entry>
       Summation is available on the following data types:
       <type>smallint</type>, <type>integer</type>,
       <type>bigint</type>, <type>real</type>, <type>double
       precision</type>, <type>numeric</type>, <type>interval</type>.
       The result is of type <type>bigint</type> for <type>smallint</type>
       or <type>integer</type> input, <type>numeric</type> for
       <type>bigint</type> 
       input, <type>double precision</type> for floating point input,
       otherwise the same as the input data type.
      </entry>
     </row>

     <row>
      <entry>VARIANCE(<replaceable class="parameter">expression</replaceable>)</entry>
      <entry>the sample variance of the input values</entry>
      <entry>
       <indexterm>
        <primary>variance</primary>
       </indexterm>
       The variance is the square of the standard deviation.  The
       supported data types and result types are the same as for
       standard deviation.
      </entry>
     </row>

    </tbody>
   </tgroup>
  </table>

  <para>
   It should be noted that except for <function>COUNT</function>,
   these functions return NULL when no rows are selected.  In
   particular, <function>SUM</function> of no rows returns NULL, not
   zero as one might expect.  <function>COALESCE</function> may be
   used to substitute zero for NULL when necessary.
  </para>

 </sect1>

</chapter>

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