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$Header: /cvsroot/pgsql/doc/src/sgml/ref/fetch.sgml,v 1.25 2003/02/04 11:23:58 momjian Exp $
PostgreSQL documentation
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<refentry id="SQL-FETCH">
 <refmeta>
  <refentrytitle id="SQL-FETCH-TITLE">FETCH</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   FETCH
  </refname>
  <refpurpose>
   retrieve rows from a table using a cursor
  </refpurpose>
 </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
FETCH [ <replaceable class="PARAMETER">direction</replaceable> ] [ <replaceable class="PARAMETER">count</replaceable> ] { IN | FROM } <replaceable class="PARAMETER">cursor</replaceable>
FETCH [ FORWARD | BACKWARD | RELATIVE ] [ <replaceable class="PARAMETER">#</replaceable> | ALL | NEXT | PRIOR ]
    { IN | FROM } <replaceable class="PARAMETER">cursor</replaceable>
  </synopsis>

  <refsect2 id="R2-SQL-FETCH-1">
   <refsect2info>
    <date>1998-09-01</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER">direction</replaceable></term>
      <listitem>
       <para>
	<replaceable class="PARAMETER">direction</replaceable>
	defines the fetch direction. It can be one of
	the following:

	<variablelist>
	 <varlistentry>
	  <term>FORWARD</term>
	  <listitem>
	   <para>
	    fetch next row(s). This is the default
	    if <replaceable class="PARAMETER">direction</replaceable> is omitted.
	   </para>
	  </listitem>
	 </varlistentry>
	 <varlistentry>
	  <term>BACKWARD</term>
	  <listitem>
	   <para>
	    fetch previous row(s).
	   </para>
	  </listitem>
	 </varlistentry>
	 <varlistentry>
	  <term>RELATIVE</term>
	  <listitem>
	   <para>
	    Noise word for SQL92 compatibility.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">count</replaceable></term>
      <listitem>
       <para>
	<replaceable class="PARAMETER">count</replaceable>
	determines how many rows to fetch. It can be one of the following:

	<variablelist>
	 <varlistentry>
	  <term><replaceable class="PARAMETER">#</replaceable></term>
	  <listitem>
	   <para>
	    A signed integer constant that specifies how many rows to fetch.
	    Note that a negative integer is equivalent to changing the sense of
	    FORWARD and BACKWARD. Zero re-fetches the current row, if any.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>
	   ALL
	  </term>
	  <listitem>
	   <para>
	    Retrieve all remaining rows.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>
	   NEXT
	  </term>
	  <listitem>
	   <para>
	    Equivalent to specifying a count of <command>1</command>.
	   </para>
	  </listitem>
	 </varlistentry>

	 <varlistentry>
	  <term>
	   PRIOR
	  </term>
	  <listitem>
	   <para>
	    Equivalent to specifying a count of <command>-1</command>.
	   </para>
	  </listitem>
	 </varlistentry>
	</variablelist>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">cursor</replaceable></term>
      <listitem>
       <para>
	An open cursor's name.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-FETCH-2">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>
    <command>FETCH</command> returns rows from the result of the query defined
    by the specified cursor.
    The following messages will be returned if the query fails:

    <variablelist>
     <varlistentry>
      <term><computeroutput>
WARNING:  PerformPortalFetch: portal "<replaceable class="PARAMETER">cursor</replaceable>" not found
       </computeroutput></term>
      <listitem>
       <para>
	If <replaceable class="PARAMETER">cursor</replaceable>
	is not previously declared.
	The cursor must be declared within a transaction block.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>
WARNING:  FETCH/ABSOLUTE not supported, using RELATIVE
       </computeroutput></term>
      <listitem>
       <para>
	<productname>PostgreSQL</productname> does not support absolute
	positioning of cursors.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-FETCH-1">
  <refsect1info>
   <date>1998-04-15</date>
  </refsect1info>
  <title>
   Description
  </title>

  <para>
   <command>FETCH</command> allows a user to retrieve rows using a cursor.
   The number of rows retrieved is specified by
   <replaceable class="PARAMETER">#</replaceable>.
   If the number of rows remaining in the cursor is less
   than <replaceable class="PARAMETER">#</replaceable>,
   then only those available are fetched.
   Substituting the keyword ALL in place of a number will
   cause all remaining rows in the cursor to be retrieved.
   Rows may be fetched in both FORWARD and BACKWARD
   directions. The default direction is FORWARD.
  </para>

  <para>
   The cursor position can be before the first row of the query result, or on
   any particular row of the result, or after the last row of the result.
   When created, a cursor is positioned before the first row.  After fetching
   some rows, the cursor is positioned on the last row retrieved.  A new
   <command>FETCH</command> always steps one row in the specified direction
   (if possible) before beginning to return rows.  If the
   <command>FETCH</command> requests more rows than available, the cursor is
   left positioned after the last row of the query result (or before the first
   row, in the case of a backward fetch).  This will always be the case after
   <command>FETCH ALL</>.
  </para>

   <tip>
    <para>
     A zero row count requests fetching the current row without moving the
     cursor --- that is, re-fetching the most recently fetched row.
     This will succeed unless the cursor is positioned before the
     first row or after the last row; in which case, no row is returned.
    </para>
   </tip>

   <tip>
    <para>
     Negative numbers are allowed to be specified for the
     row count. A negative number is equivalent to reversing
     the sense of the FORWARD and BACKWARD keywords. For example,
     <command>FORWARD -1</command> is the same as <command>BACKWARD 1</command>.
    </para>
   </tip>

  <refsect2 id="R2-SQL-FETCH-3">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Notes
   </title>

   <para>
    Note that the FORWARD, BACKWARD, and ALL keywords are
    <productname>PostgreSQL</productname> extensions.
    See below for details on compatibility issues.
   </para>

   <para>
    Updating data in a cursor is not supported by 
    <productname>PostgreSQL</productname>,
    because mapping cursor updates back to base tables is
    not generally possible, as is also the case with VIEW updates.
    Consequently,
    users must issue explicit UPDATE commands to replace data.
   </para>

   <para>
    Cursors may only be used inside of transactions because
    the data that they store spans multiple user queries.
   </para>

   <para>
    <xref linkend="sql-declare" endterm="sql-declare-title">
    is used to define a cursor.
    Use
    <xref linkend="sql-move" endterm="sql-move-title">
    to change cursor position without retrieving data.
    Refer to
    <xref linkend="sql-begin" endterm="sql-begin-title">,
    <xref linkend="sql-commit" endterm="sql-commit-title">,
    and
    <xref linkend="sql-rollback" endterm="sql-rollback-title">
    for further information about transactions.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-FETCH-2">
  <title>
   Usage
  </title>

  <para>
   The following examples traverses a table using a cursor.

<programlisting>
-- Set up and use a cursor:

BEGIN WORK;
DECLARE liahona CURSOR FOR SELECT * FROM films;

-- Fetch first 5 rows in the cursor liahona:
FETCH FORWARD 5 IN liahona;

<computeroutput>
 code  |          title          | did | date_prod  |  kind    | len
-------+-------------------------+-----+------------+----------+-------
 BL101 | The Third Man           | 101 | 1949-12-23 | Drama    | 01:44
 BL102 | The African Queen       | 101 | 1951-08-11 | Romantic | 01:43
 JL201 | Une Femme est une Femme | 102 | 1961-03-12 | Romantic | 01:25
 P_301 | Vertigo                 | 103 | 1958-11-14 | Action   | 02:08
 P_302 | Becket                  | 103 | 1964-02-03 | Drama    | 02:28
</computeroutput>

-- Fetch previous row:
FETCH BACKWARD 1 IN liahona;

<computeroutput>
 code  | title   | did | date_prod  | kind   | len
-------+---------+-----+------------+--------+-------
 P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08
</computeroutput>

-- close the cursor and commit work:

CLOSE liahona;
COMMIT WORK;
</programlisting>
  </para>        
 </refsect1>

 <refsect1 id="R1-SQL-FETCH-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-FETCH-4">
   <refsect2info>
    <date>1998-09-01</date>
   </refsect2info>
   <title>
    SQL92
   </title>

   <para>
    <note>
     <para>
      The non-embedded use of cursors is a <productname>PostgreSQL</productname>
      extension. The syntax and usage of cursors is being compared
      against the embedded form of cursors defined in <acronym>SQL92</acronym>.
     </para>
    </note>
   </para>

   <para>
    <acronym>SQL92</acronym> allows absolute positioning of the cursor for
    FETCH, and allows placing the results into explicit variables:

    <synopsis>
FETCH ABSOLUTE <replaceable class="PARAMETER">#</replaceable>
    FROM <replaceable class="PARAMETER">cursor</replaceable>
    INTO :<replaceable class="PARAMETER">variable</replaceable> [, ...]
    </synopsis>

    <variablelist>
     <varlistentry>
      <term>ABSOLUTE</term>
      <listitem>
       <para>
	The cursor should be positioned to the specified absolute
	row number. All row numbers in <productname>PostgreSQL</productname>
	are relative numbers so this capability is not supported.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>:<replaceable class="PARAMETER">variable</replaceable></term>
      <listitem>
       <para>
	Target host variable(s).
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsect1>
</refentry>

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