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<chapter id="sql-syntax">
 <title>SQL Syntax</title>

  <abstract>
   <para>
    A description of the general syntax of SQL.
   </para>
  </abstract>

 <sect1 id="sql-syntax-lexical">
  <title>Lexical Structure</title>

  <para>
   SQL input consists of a sequence of
   <firstterm>commands</firstterm>.  A command is composed of a
   sequence of <firstterm>tokens</firstterm>, which depend on the
   syntax of the particular command, terminated by a semicolon
   (<quote>;</quote>).  The end of the input stream also terminates a
   command.
  </para>

  <para>
   A token can be a <firstterm>key word</firstterm>, an
   <firstterm>identifier</firstterm>, a <firstterm>quoted
   identifier</firstterm>, a <firstterm>literal</firstterm> (or
   constant), or a special character symbol.  Tokens are normally
   separated by whitespace (space, tab, newline), but need not be if
   there is no ambiguity (which is generally only the case if a
   special character is adjacent to some other token type).
  </para>

  <para>
   Additionally, <firstterm>comments</firstterm> can occur in SQL
   input.  They are not tokens, they are effectively equivalent to
   whitespace.
  </para>

  <informalexample id="sql-syntax-ex-commands">
   <para>
    For example, the following is (lexically) valid SQL input:
<programlisting>
SELECT * FROM MY_TABLE;
UPDATE MY_TABLE SET A = 5;
INSERT INTO MY_TABLE VALUES (3, 'hi there');
</programlisting>
    This is a sequence of three commands, one per line (although this
    is not required; more than one command can be on a line, and
    commands can be usefully split across lines).
   </para>
  </informalexample>

  <para>
   The SQL syntax is not very consistent regarding what tokens
   identify commands and which are operands or parameters.  The first
   few tokens are generally the command name, so in the above example
   we would usually speak of a <quote>SELECT</quote>, an
   <quote>UPDATE</quote>, and an <quote>INSERT</quote> command.  But
   for instance the <command>UPDATE</command> command always requires
   a <token>SET</token> token to appear in a certain position, and
   this particular variation of <command>INSERT</command> also
   requires a <token>VALUES</token> in order to be complete.  The
   precise syntax rules for each command are described in the
   <citetitle>Reference Manual</citetitle>.
  </para>

  <sect2 id="sql-syntax-identifiers">
   <title>Identifiers and Key Words</title>

   <para>
    Tokens such as <token>SELECT</token>, <token>UPDATE</token>, or
    <token>VALUES</token> in the example above are examples of
    <firstterm>key words</firstterm>, that is, words that have a fixed
    meaning in the SQL language.  The tokens <token>MY_TABLE</token>
    and <token>A</token> are examples of
    <firstterm>identifiers</firstterm>.  They identify names of
    tables, columns, or other database objects, depending on the
    command they are used in.  Therefore they are sometimes simply
    called <quote>names</quote>.  Key words and identifiers have the
    same lexical structure, meaning that one cannot know whether a
    token is an identifier or a key word without knowing the language.
    A complete list of key words can be found in <xref
    linkend="sql-keywords-appendix">.
   </para>

   <para>
    SQL identifiers and key words must begin with a letter
    (<literal>a</literal>-<literal>z</literal>) or underscore
    (<literal>_</literal>).  Subsequent characters in an identifier or
    key word can be letters, digits
    (<literal>0</literal>-<literal>9</literal>), or underscores,
    although the SQL standard will not define a key word that contains
    digits or start or ends with an underscore.
   </para>

   <para>
    The system uses no more than <symbol>NAMEDATALEN</symbol>-1
    characters of an identifier; longer names can be written in
    commands, but they will be truncated.  By default,
    <symbol>NAMEDATALEN</symbol> is 32 so the maximum identifier length
    is 31 (but at the time the system is built,
    <symbol>NAMEDATALEN</symbol> can be changed in
    <filename>src/include/postgres_ext.h</filename>).
   </para>

   <para>
    Identifier and key word names are case insensitive.  Therefore
<programlisting>
UPDATE MY_TABLE SET A = 5;
</programlisting>
    can equivalently be written as
<programlisting>
uPDaTE my_TabLE SeT a = 5;
</programlisting>
    A good convention to adopt is perhaps to write key words in upper
    case and names in lower case, e.g.,
<programlisting>
UPDATE my_table SET a = 5;
</programlisting>
   </para>

   <para>
    There is a second kind of identifier:  the <firstterm>delimited
    identifier</firstterm> or <firstterm>quoted
    identifier</firstterm>.  It is formed by enclosing an arbitrary
    sequence of characters in double-quotes
    (<literal>"</literal>). <!-- " font-lock mania --> A delimited
    identifier is always an identifier, never a key word.  So
    <literal>"select"</literal> could be used to refer to a column or
    table named <quote>select</quote>, whereas an unquoted
    <literal>select</literal> would be taken as part of a command and
    would therefore provoke a parse error when used where a table or
    column name is expected.  The example can be written with quoted
    identifiers like so:
<programlisting>
UPDATE "my_table" SET "a" = 5;
</programlisting>
   </para>

   <para>
    Quoted identifiers can contain any character other than a double
    quote itself.  This allows constructing table or column names that
    would otherwise not be possible, such as ones containing spaces or
    ampersands.  The length limitation still applies.
   </para>

   <para>
    Quoting an identifier also makes it case-sensitive, whereas
    unquoted names are always folded to lower case.  For example, the
    identifiers <literal>FOO</literal>, <literal>foo</literal> and
    <literal>"foo"</literal> are considered the same by
    <productname>Postgres</productname>, but <literal>"Foo"</literal>
    and <literal>"FOO"</literal> are different from these three and
    each other.
    <footnote>
     <para>
      This is incompatible with SQL, where unquoted names are folded to
      upper case.  Thus, <literal>foo</literal> is equivalent to
      <literal>"FOO"</literal>.  If you want to write portable
      applications you are advised to always quote a particular name or
      never quote it.
     </para>
    </footnote>
   </para>
  </sect2>


  <sect2 id="sql-syntax-constants">
   <title>Constants</title>

   <para>
    There are four kinds of <firstterm>implicitly typed
    constants</firstterm> in <productname>Postgres</productname>:
    strings, bit strings, integers, and floating point numbers.
    Constants can also be specified with explicit types, which can
    enable more accurate representation and more efficient handling by
    the system. The implicit constants are described below; explicit
    constants are discussed afterwards.
   </para>

   <sect3>
    <title>String Constants</title>

    <para>
     A string constant in SQL is an arbitrary sequence of characters
     bounded by single quotes (<quote>'</quote>), e.g., <literal>'This
     is a string'</literal>.  SQL allows single quotes to be embedded
     in strings by typing two adjacent single quotes (e.g.,
     <literal>'Dianne''s horse'</literal>).  In
     <productname>Postgres</productname> single quotes may
     alternatively be escaped with a backslash (<quote>\</quote>,
     e.g., <literal>'Dianne\'s horse'</literal>).
    </para>

    <para>
     C-style backslash escapes are also available:
     <literal>\b</literal> is a backspace, <literal>\f</literal> is a
     form feed, <literal>\n</literal> is a newline,
     <literal>\r</literal> is a carriage return, <literal>\t</literal>
     is a tab, and <literal>\<replaceable>xxx</replaceable></literal>,
     where <replaceable>xxx</replaceable> is an octal number, is the
     character with the corresponding ASCII code.  Any other character
     following a backslash is taken literally.  Thus, to include a
     backslash in a string constant, type two backslashes.
    </para>

    <para>
     The character with the code zero cannot be in a string constant.
    </para>

    <para>
     Two string constants that are only separated by whitespace
     <emphasis>with at least one newline</emphasis> are concatenated
     and effectively treated as if the string had been written in one
     constant.  For example:
<programlisting>
SELECT 'foo'
'bar';
</programlisting>
     is equivalent to
<programlisting>
SELECT 'foobar';
</programlisting>
     but
<programlisting>
SELECT 'foo'      'bar';
</programlisting>
     is not valid syntax.
    </para>
   </sect3>

   <sect3>
    <title>Bit String Constants</title>

    <para>
     Bit string constants look like string constants with a
     <literal>B</literal> (upper or lower case) immediately before the
     opening quote (no intervening whitespace), e.g.,
     <literal>B'1001'</literal>.  The only characters allowed within
     bit string constants are <literal>0</literal> and
     <literal>1</literal>.  Bit strings constants can be continued
     across lines in the same way as regular string constants.
    </para>
   </sect3>

   <sect3>
    <title>Integer Constants</title>

    <para>
     Integer constants in SQL are sequences of decimal digits (0
     though 9) with no decimal point.  The range of legal values
     depends on which integer data type is used, but the plain
     <type>integer</type> type accepts values ranging from -2147483648
     to +2147483647.  (The optional plus or minus sign is actually a
     separate unary operator and not part of the integer constant.)
    </para>
   </sect3>

   <sect3>
    <title>Floating Point Constants</title>

    <para>
     Floating point constants are accepted in these general forms:
<synopsis>
<replaceable>digits</replaceable>.<optional><replaceable>digits</replaceable></optional><optional>e<optional>+-</optional><replaceable>digits</replaceable></optional>
<optional><replaceable>digits</replaceable></optional>.<replaceable>digits</replaceable><optional>e<optional>+-</optional><replaceable>digits</replaceable></optional>
<replaceable>digits</replaceable>e<optional>+-</optional><replaceable>digits</replaceable>
</synopsis>
     where <replaceable>digits</replaceable> is one or more decimal
     digits.  At least one digit must be before or after the decimal
     point and after the <literal>e</literal> if you use that option.
     Thus, a floating point constant is distinguished from an integer
     constant by the presence of either the decimal point or the
     exponent clause (or both).  There must not be a space or other
     characters embedded in the constant.
    </para>

    <informalexample>
     <para>
      These are some examples of valid floating point constants:
<literallayout>
3.5
4.
.001
5e2
1.925e-3
</literallayout>
     </para>
    </informalexample>

    <para>
     Floating point constants are of type <type>DOUBLE
     PRECISION</type>. <type>REAL</type> can be specified explicitly
     by using <acronym>SQL</acronym> string notation or
     <productname>Postgres</productname> type notation:

<programlisting>
REAL '1.23'  -- string style
'1.23'::REAL -- Postgres (historical) style
     </programlisting>
    </para>
   </sect3>

   <sect3>
    <title>Constants of Other Types</title>

    <para>
     A constant of an <emphasis>arbitrary</emphasis> type can be
     entered using any one of the following notations:
<synopsis>
<replaceable>type</replaceable> '<replaceable>string</replaceable>'
'<replaceable>string</replaceable>'::<replaceable>type</replaceable>
CAST ( '<replaceable>string</replaceable>' AS <replaceable>type</replaceable> )
</synopsis>
     The value inside the string is passed to the input conversion
     routine for the type called <replaceable>type</replaceable>. The
     result is a constant of the indicated type.  The explicit type
     cast may be omitted if there is no ambiguity as to the type the
     constant must be (for example, when it is passed as an argument
     to a non-overloaded function), in which case it is automatically
     coerced.
    </para>

    <para>
     It is also possible to specify a type coercion using a function-like
     syntax:
<synopsis>
<replaceable>typename</replaceable> ( <replaceable>value</replaceable> )
</synopsis>
     although this only works for types whose names are also valid as
     function names.  (For example, <literal>double precision</literal>
     can't be used this way --- but the equivalent <literal>float8</literal>
     can.)
    </para>

    <para>
     The <literal>::</literal>, <literal>CAST()</literal>, and
     function-call syntaxes can also be used to specify the type of
     arbitrary expressions, but the form
     <replaceable>type</replaceable>
     '<replaceable>string</replaceable>' can only be used to specify
     the type of a literal constant.
    </para>
   </sect3>

   <sect3>
    <title>Array constants</title>

    <para>
     The general format of an array constant is the following:
<synopsis>
'{ <replaceable>val1</replaceable> <replaceable>delim</replaceable> <replaceable>val2</replaceable> <replaceable>delim</replaceable> ... }'
</synopsis>
     where <replaceable>delim</replaceable> is the delimiter character
     for the type, as recorded in its <literal>pg_type</literal>
     entry.  (For all built-in types, this is the comma character
     ",".)  Each <replaceable>val</replaceable> is either a constant
     of the array element type, or a sub-array.  An example of an
     array constant is
<programlisting>
'{{1,2,3},{4,5,6},{7,8,9}}'
</programlisting>
     This constant is a two-dimensional, 3 by 3 array consisting of three
     sub-arrays of integers.
    </para>

    <para>
     Individual array elements can be placed between double-quote
     marks (<literal>"</literal>) <!-- " --> to avoid ambiguity
     problems with respect to white space.  Without quote marks, the
     array-value parser will skip leading white space.
    </para>

    <para>
     (Array constants are actually only a special case of the generic
     type constants discussed in the previous section.  The constant
     is initially treated as a string and passed to the array input
     conversion routine.  An explicit type specification might be
     necessary.)
    </para>
   </sect3>
  </sect2>


  <sect2 id="sql-syntax-operators">
   <title>Operators</title>

   <para>
    An operator is a sequence of up to <symbol>NAMEDATALEN</symbol>-1
    (31 by default) characters from the following list:
<literallayout>
+ - * / &lt; &gt; = ~ ! @ # % ^ &amp; | ` ? $
</literallayout>

    There are a few restrictions on operator names, however:
    <itemizedlist>
     <listitem>
      <para>
       "$" (dollar) cannot be a single-character operator, although it
       can be part of a multi-character operator name.
      </para>
     </listitem>

     <listitem>
      <para>
       <literal>--</literal> and <literal>/*</literal> cannot appear
       anywhere in an operator name, since they will be taken as the
       start of a comment.
      </para>
     </listitem>

     <listitem>
      <para>
       A multi-character operator name cannot end in "+" or "-",
       unless the name also contains at least one of these characters:
<literallayout>
~ ! @ # % ^ &amp; | ` ? $
</literallayout>
       For example, <literal>@-</literal> is an allowed operator name,
       but <literal>*-</literal> is not.  This restriction allows
       <productname>Postgres</productname> to parse SQL-compliant
       queries without requiring spaces between tokens.
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    When working with non-SQL-standard operator names, you will usually
    need to separate adjacent operators with spaces to avoid ambiguity.
    For example, if you have defined a left-unary operator named "@",
    you cannot write <literal>X*@Y</literal>; you must write
    <literal>X* @Y</literal> to ensure that
    <productname>Postgres</productname> reads it as two operator names
    not one.
   </para>
  </sect2>

  <sect2>
   <title>Special Characters</title>

  <para>
   Some characters that are not alphanumeric have a special meaning
   that is different from being an operator.  Details on the usage can
   be found at the location where the respective syntax element is
   described.  This section only exists to advise the existence and
   summarize the purposes of these characters.

   <itemizedlist>
    <listitem>
     <para>
      A dollar sign (<literal>$</literal>) followed by digits is used
      to represent the positional parameters in the body of a function
      definition.  In other contexts the dollar sign may be part of an
      operator name.
     </para>
    </listitem>

    <listitem>
     <para>
      Parentheses (<literal>()</literal>) have their usual meaning to
      group expressions and enforce precedence.  In some cases
      parentheses are required as part of the fixed syntax of a
      particular SQL command.
     </para>
    </listitem>

    <listitem>
     <para>
      Brackets (<literal>[]</literal>) are used to select the elements
      of an array.  See <xref linkend="arrays"> for more information
      on arrays.
     </para>
    </listitem>

    <listitem>
     <para>
      Commas (<literal>,</literal>) are used in some syntactical
      constructs to separate the elements of a list.
     </para>
    </listitem>

    <listitem>
     <para>
      The semicolon (<literal>;</literal>) terminates an SQL command.
      It cannot appear anywhere within a command, except when quoted
      as a string constant or identifier.
     </para>
    </listitem>

    <listitem>
     <para>
      The colon (<literal>:</literal>) is used to select
      <quote>slices</quote> from arrays. (See <xref
      linkend="arrays">.)  In certain SQL dialects (such as Embedded
      SQL), the colon is used to prefix variable names.
     </para>
    </listitem>

    <listitem>
     <para>
      The asterisk (<literal>*</literal>) has a special meaning when
      used in the <command>SELECT</command> command or with the
      <function>COUNT</function> aggregate function.
     </para>
    </listitem>

    <listitem>
     <para>
      The period (<literal>.</literal>) is used in floating point
      constants, and to separate table and column names.
     </para>
    </listitem>
   </itemizedlist>

   </para>
  </sect2>

  <sect2 id="sql-syntax-comments">
   <title>Comments</title>

   <para>
    A comment is an arbitrary sequence of characters beginning with
    double dashes and extending to the end of the line, e.g.:
<programlisting>
-- This is a standard SQL92 comment
</programlisting>
   </para>

   <para>
    Alternatively, C-style block comments can be used:
<programlisting>
/* multi-line comment
 * with nesting: /* nested block comment */
 */
</programlisting>
    where the comment begins with <literal>/*</literal> and extends to
    the matching occurrence of <literal>*/</literal>. These block
    comments nest, as specified in SQL99 but unlike C, so that one can
    comment out larger blocks of code that may contain existing block
    comments.
   </para>

   <para>
    A comment is removed from the input stream before further syntax
    analysis and is effectively replaced by whitespace.
   </para>
  </sect2>
 </sect1>


  <sect1 id="sql-syntax-columns">
   <title>Fields and Columns</title>

   <sect2>
    <title>Fields</title>

    <para>
     A <firstterm>field</firstterm>
     is either a user-defined attribute of a given class or one of the
     following system-defined attributes:

     <variablelist>
      <varlistentry>
       <term>oid</term>
       <listitem>
	<para>
	 The unique identifier (object ID) of a row.  This is a serial number
	 that is added by Postgres to all rows automatically. OIDs are not
	 reused and are 32-bit quantities.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term>tableoid</term>
       <listitem>
	<para>
	 The OID of the table containing this row.  This attribute is
	 particularly handy for queries that select from inheritance
	 hierarchies, since without it, it's difficult to tell which
	 individual table a row came from.  The tableoid can be joined
	 against the OID attribute of pg_class to obtain the table name.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>xmin</term>
       <listitem>
	<para>
	 The identity (transaction ID) of the inserting transaction for
	 this tuple.  (Note: a tuple is an individual state of a row;
	 each UPDATE of a row creates a new tuple for the same logical row.)
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term>cmin</term>
       <listitem>
	<para>
	 The command identifier (starting at zero) within the inserting
	 transaction.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term>xmax</term>
       <listitem>
	<para>
	 The identity (transaction ID) of the deleting transaction,
	 or zero for an undeleted tuple.  In practice, this is never nonzero
	 for a visible tuple.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term>cmax</term>
       <listitem>
	<para>
	 The command identifier within the deleting transaction, or zero.
	 Again, this is never nonzero for a visible tuple.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
      <term>ctid</term>
       <listitem>
	<para>
	 The tuple ID of the tuple within its table.  This is a pair
	 (block number, tuple index within block) that identifies the
	 physical location of the tuple.  Note that although the ctid
	 can be used to locate the tuple very quickly, a row's ctid
	 will change each time it is updated or moved by VACUUM.
	 Therefore ctid is useless as a long-term row identifier.
	 The OID, or even better a user-defined serial number, should
	 be used to identify logical rows.
	</para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

    <para>
     For further information on the system attributes consult
     <xref linkend="STON87a" endterm="STON87a">.
     Transaction and command identifiers are 32 bit quantities.
    </para>
   </sect2>

   <sect2>
    <title>Columns</title>

    <para>
     A <firstterm>column</firstterm> is a construct of the form:

     <synopsis>
<replaceable>instance</replaceable>{.<replaceable>composite_field</replaceable>}.<replaceable>field</replaceable> `['<replaceable>subscript</replaceable>`]'
     </synopsis>

     <replaceable>instance</replaceable>
     identifies a particular class and can be thought of as standing for
     the instances of that class.  An instance variable is either a class
     name, an alias for a class defined by means of a FROM clause,
     or the keyword NEW or OLD.
     (NEW and OLD can only appear in the action portion of a rule, while
     other instance variables can be used in any SQL statement.)  The
     instance name can be omitted if the first field name is unique
     across all the classes being used in the current query.
     <replaceable>composite_field</replaceable>
     is a field of of one of the Postgres composite types,
     while successive composite fields select attributes in the
     class(s) to which the composite field evaluates.  Lastly,
     <replaceable>field</replaceable>
     is a normal (base type) field in the class(s) last addressed.  If
     <replaceable>field</replaceable>
     is of an array type,
     then the optional <replaceable>subscript</replaceable>
     selects a specific element in the array.  If no subscript is
     provided, then the whole array is selected.
    </para>
   </sect2>
  </sect1>

  <sect1 id="sql-expressions">
   <title>Expressions</title>

   <para>
    <acronym>SQL92</acronym> allows <firstterm>expressions</firstterm>
    to transform data in tables. Expressions may contain operators
    and functions.
   </para>

   <para>
    An expression is one of the following:

    <simplelist>
     <member>constant</member>
     <member>column</member>
     <member><replaceable>expression</replaceable> <replaceable>binary_operator</replaceable> <replaceable>expression</replaceable></member>
     <member><replaceable>expression</replaceable> <replaceable>right_unary_operator</replaceable></member>
     <member><replaceable>left_unary_operator</replaceable> <replaceable>expression</replaceable></member>
     <member>( <replaceable>expression</replaceable> )</member>
     <member>parameter</member>
     <member>functional expression</member>
     <member>aggregate expression</member>
    </simplelist>
   </para>

   <para>
    We have already discussed constants and columns.  The three kinds of
    operator expressions indicate respectively binary (infix), right-unary
    (suffix) and left-unary (prefix) operators.  The following sections
    discuss the remaining options.
   </para>

   <sect2>
    <title>Parameters</title>

    <para>
     A <firstterm>parameter</firstterm>
     is used to indicate a parameter in a SQL function.  Typically this
     is used in SQL function definition statements.  The form of a
     parameter is:

     <synopsis>
$<replaceable class="parameter">number</replaceable>
     </synopsis>
    </para>

    <para>
     For example, consider the definition of a function,
     <function>dept</function>, as

     <programlisting>
CREATE FUNCTION dept (name)
	RETURNS dept
	AS 'select * from dept where name = $1'
	LANGUAGE 'sql';
     </programlisting>
    </para>
   </sect2>

   <sect2>
    <title>Functional Expressions</title>

    <para>
     A <firstterm>functional expression</firstterm>
     is the name of a legal SQL function, followed by its argument list
     enclosed in parentheses:

     <synopsis>
<replaceable>function</replaceable> (<replaceable>expression</replaceable> [, <replaceable>expression</replaceable> ... ] )
     </synopsis>
    </para>

    <para>
     For example, the following computes the square root of an employee
     salary:

     <programlisting>
sqrt(emp.salary)
     </programlisting>
    </para>
   </sect2>

   <sect2 id="syntax-aggregates">
    <title>Aggregate Expressions</title>

    <para>
     An <firstterm>aggregate expression</firstterm> represents the application
     of an aggregate function across the rows selected by a query.
     An aggregate function reduces multiple inputs to a single output value,
     such as the sum or average of the inputs.
     The syntax of an aggregate expression is one of the following:

    <simplelist>
     <member><replaceable>aggregate_name</replaceable> (<replaceable>expression</replaceable>)</member>
     <member><replaceable>aggregate_name</replaceable> (ALL <replaceable>expression</replaceable>)</member>
     <member><replaceable>aggregate_name</replaceable> (DISTINCT <replaceable>expression</replaceable>)</member>
     <member><replaceable>aggregate_name</replaceable> ( * )</member>
    </simplelist>

     where <replaceable>aggregate_name</replaceable> is a previously defined
     aggregate, and <replaceable>expression</replaceable> is any expression
     that doesn't itself contain an aggregate expression.
    </para>

    <para>
     The first form of aggregate expression invokes the aggregate across all
     input rows for which the given expression yields a non-null value.
     The second form is the same as the first, since ALL is the default.
     The third form invokes the aggregate for all distinct non-null values
     of the expression found in the input rows.  The last form invokes the
     aggregate once for each input row regardless of null or non-null values;
     since no particular input value is specified, it is generally only useful
     for the count() aggregate.
    </para>

    <para>
      For example, count(*) yields the total number of input rows;
      count(f1) yields the number of input rows in which f1 is non-null;
      count(distinct f1) yields the number of distinct non-null values of f1.
    </para>
   </sect2>

   <sect2>
    <title>Target List</title>

    <para>
     A <firstterm>target list</firstterm>
     is a comma-separated list of one or more elements, each
     of which must be of the form:

     <synopsis>
<replaceable>expression</replaceable> [ AS <replaceable>result_attname</replaceable> ]
     </synopsis>

     where <replaceable>result_attname</replaceable>
     is the name to be assigned to the created column.  If 
     <replaceable>result_attname</replaceable>
     is not present, then <productname>Postgres</productname> selects a
     default name based on the contents of <replaceable>expression</replaceable>.
     If <replaceable>expression</replaceable> is a simple attribute reference
     then the default name will be the same as that attribute's name, but
     otherwise the implementation is free to assign any default name.
    </para>
   </sect2>

   <sect2>
    <title>Qualification</title>

    <para>
     A <firstterm>qualification</firstterm>
     consists of any number of clauses connected by the logical operators:

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

     A clause is an <replaceable>expression</replaceable>
     that evaluates to a <literal>boolean</literal> over a set of instances.
    </para>
   </sect2>

   <sect2>
    <title>From List</title>

    <para>
     The <firstterm>from list</firstterm>
     is a comma-separated list of <firstterm>from-expressions</firstterm>.
     The simplest possibility for a from-expression is:

     <synopsis>
<replaceable>class_reference</replaceable> [ [ AS ] <replaceable class="PARAMETER">alias</replaceable> ]
     </synopsis>

     where <replaceable>class_reference</replaceable> is of the form

     <synopsis>
[ ONLY ] <replaceable class="PARAMETER">table_name</replaceable> [ * ]
     </synopsis>

     The from-expression defines an instance variable that ranges over the
     rows of the specified table.  The instance variable's name is either
     the table name, or the <replaceable>alias</replaceable> if one is given.
     Ordinarily, if the table has child tables then the instance variable
     will range over all rows in the inheritance hierarchy starting with
     the specified table.  If <literal>ONLY</literal> is specified then
     child tables are not included.  A trailing asterisk <literal>*</literal>
     can be written to specifically indicate that child tables are included
     (<literal>ONLY</literal> and <literal>*</literal> are mutually
     exclusive).
    </para>

    <para>
     A from-expression can also be a sub-query:

     <synopsis>
( <replaceable class="PARAMETER">select-statement</replaceable> ) [ AS ] <replaceable class="PARAMETER">alias</replaceable>
     </synopsis>

     Here, the effect is as though the SELECT were executed and its results
     stored in a temporary table, which then becomes available as an instance
     variable under the given <replaceable>alias</replaceable>.
    </para>

    <para>
     Finally, a from-expression can be built up from simpler from-expressions
     using JOIN clauses:

     <synopsis>
<replaceable class="PARAMETER">from_expression</replaceable> [ NATURAL ] <replaceable class="PARAMETER">join_type</replaceable> <replaceable class="PARAMETER">from_expression</replaceable>
    [ ON <replaceable class="PARAMETER">join_condition</replaceable> | USING ( <replaceable class="PARAMETER">join_column_list</replaceable> ) ]
     </synopsis>

     This syntax allows specification of <firstterm>outer joins</firstterm>.
     For details see the reference page for SELECT.
    </para>
   </sect2>


  <sect2 id="sql-precedence">
   <title>Lexical Precedence</title>

   <para>
    The precedence and associativity of the operators is hard-wired
    into the parser.  Most operators have the same precedence and are
    left-associative.  This may lead to non-intuitive behavior; for
    example the boolean operators "&lt;" and "&gt;" have a different
    precedence than the boolean operators "&lt;=" and "&gt;=".  Also,
    you will sometimes need to add parentheses when using combinations
    of binary and unary operators.  For instance
<programlisting>
SELECT 5 &amp; ~ 6;
</programlisting>
   will be parsed as
<programlisting>
SELECT (5 &amp;) ~ 6;
</programlisting>
    because the parser has no idea that <token>&amp;</token> is
    defined as a binary operator.  This is the price one pays for
    extensibility.
   </para>

   <table tocentry="1">
    <title>Operator Ordering (decreasing precedence)</title>

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

     <tbody>
      <row>
       <entry><token>::</token></entry>
       <entry>left</entry>
       <entry><productname>Postgres</productname>-style typecast</entry>
      </row>

      <row>
       <entry><token>[</token> <token>]</token></entry>
       <entry>left</entry>
       <entry>array element selection</entry>
      </row>

      <row>
       <entry><token>.</token></entry>
       <entry>left</entry>
       <entry>table/column name separator</entry>
      </row>

      <row>
       <entry><token>-</token></entry>
       <entry>right</entry>
       <entry>unary minus</entry>
      </row>

      <row>
       <entry><token>^</token></entry>
       <entry>left</entry>
       <entry>exponentiation</entry>
      </row>

      <row>
       <entry><token>*</token> <token>/</token> <token>%</token></entry>
       <entry>left</entry>
       <entry>multiplication, division, modulo</entry>
      </row>

      <row>
       <entry><token>+</token> <token>-</token></entry>
       <entry>left</entry>
       <entry>addition, subtraction</entry>
      </row>

      <row>
       <entry><token>IS</token></entry>
       <entry></entry>
       <entry>test for TRUE, FALSE, NULL</entry>
      </row>

      <row>
       <entry><token>ISNULL</token></entry>
       <entry></entry>
       <entry>test for NULL</entry>
      </row>

      <row>
       <entry><token>NOTNULL</token></entry>
       <entry></entry>
       <entry>test for NOT NULL</entry>
      </row>

      <row>
       <entry>(any other)</entry>
       <entry>left</entry>
       <entry>all other native and user-defined operators</entry>
      </row>

      <row>
       <entry><token>IN</token></entry>
       <entry></entry>
       <entry>set membership</entry>
      </row>

      <row>
       <entry><token>BETWEEN</token></entry>
       <entry></entry>
       <entry>containment</entry>
      </row>

      <row>
       <entry><token>OVERLAPS</token></entry>
       <entry></entry>
       <entry>time interval overlap</entry>
      </row>

      <row>
       <entry><token>LIKE</token> <token>ILIKE</token></entry>
       <entry></entry>
       <entry>string pattern matching</entry>
      </row>

      <row>
       <entry><token>&lt;</token> <token>&gt;</token></entry>
       <entry></entry>
       <entry>less than, greater than</entry>
      </row>

      <row>
       <entry><token>=</token></entry>
       <entry>right</entry>
       <entry>equality, assignment</entry>
      </row>

      <row>
       <entry><token>NOT</token></entry>
       <entry>right</entry>
       <entry>logical negation</entry>
      </row>

      <row>
       <entry><token>AND</token></entry>
       <entry>left</entry>
       <entry>logical conjunction</entry>
      </row>

      <row>
       <entry><token>OR</token></entry>
       <entry>left</entry>
       <entry>logical disjunction</entry>
      </row>
     </tbody>
    </tgroup>
   </table>

   <para>
    Note that the operator precedence rules also apply to user-defined
    operators that have the same names as the built-in operators
    mentioned above.  For example, if you define a
    <quote>+</quote> operator for some custom data type it will have
    the same precedence as the built-in <quote>+</quote> operator, no
    matter what yours does.
   </para>
  </sect2>

  </sect1>
 </chapter>

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