fetch.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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$PostgreSQL: pgsql/doc/src/sgml/ref/fetch.sgml,v 1.38 2005/01/04 00:39:53 tgl 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 query using a cursor</refpurpose>
 </refnamediv>

 <indexterm zone="sql-fetch">
  <primary>FETCH</primary>
 </indexterm>

 <indexterm zone="sql-fetch">
  <primary>cursor</primary>
  <secondary>FETCH</secondary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
FETCH [ <replaceable class="PARAMETER">direction</replaceable> { FROM | IN } ] <replaceable class="PARAMETER">cursorname</replaceable>

where <replaceable class="PARAMETER">direction</replaceable> can be empty or one of:

    NEXT
    PRIOR
    FIRST
    LAST
    ABSOLUTE <replaceable class="PARAMETER">count</replaceable>
    RELATIVE <replaceable class="PARAMETER">count</replaceable>
    <replaceable class="PARAMETER">count</replaceable>
    ALL
    FORWARD
    FORWARD <replaceable class="PARAMETER">count</replaceable>
    FORWARD ALL
    BACKWARD
    BACKWARD <replaceable class="PARAMETER">count</replaceable>
    BACKWARD ALL
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>FETCH</command> retrieves rows using a previously-created cursor.
  </para>

  <para>
   A cursor has an associated position, which is used by
   <command>FETCH</>.  The cursor position can be before the first row of the
   query result, 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 row most recently
   retrieved.  If <command>FETCH</> runs off the end of the available rows
   then the cursor is left positioned after the last row, or before the first
   row if fetching backward.  <command>FETCH ALL</> or <command>FETCH BACKWARD
   ALL</> will always leave the cursor positioned after the last row or before
   the first row.
  </para>

  <para>
   The forms <literal>NEXT</>, <literal>PRIOR</>, <literal>FIRST</>,
   <literal>LAST</>, <literal>ABSOLUTE</>, <literal>RELATIVE</> fetch
   a single row after moving the cursor appropriately.  If there is no
   such row, an empty result is returned, and the cursor is left
   positioned before the first row or after the last row as
   appropriate.
  </para>

  <para>
   The forms using <literal>FORWARD</> and <literal>BACKWARD</>
   retrieve the indicated number of rows moving in the forward or
   backward direction, leaving the cursor positioned on the
   last-returned row (or after/before all rows, if the <replaceable
   class="PARAMETER">count</replaceable> exceeds the number of rows
   available).
  </para>

  <para>
   <literal>RELATIVE 0</>, <literal>FORWARD 0</>, and
   <literal>BACKWARD 0</> all request 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>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

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

      <variablelist>
       <varlistentry>
        <term><literal>NEXT</literal></term>
        <listitem>
         <para>
          Fetch the next row. This is the default if <replaceable
          class="PARAMETER">direction</replaceable> is omitted.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>PRIOR</literal></term>
        <listitem>
         <para>
          Fetch the prior row.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>FIRST</literal></term>
        <listitem>
         <para>
          Fetch the first row of the query (same as <literal>ABSOLUTE 1</literal>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>LAST</literal></term>
        <listitem>
         <para>
          Fetch the last row of the query (same as <literal>ABSOLUTE -1</literal>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>ABSOLUTE <replaceable class="PARAMETER">count</replaceable></literal></term>
        <listitem>
         <para>
          Fetch the <replaceable
          class="PARAMETER">count</replaceable>'th row of the query,
          or the <literal>abs(<replaceable
          class="PARAMETER">count</replaceable>)</literal>'th row from
          the end if <replaceable
          class="PARAMETER">count</replaceable> is negative.  Position
          before first row or after last row if <replaceable
          class="PARAMETER">count</replaceable> is out of range; in
          particular, <literal>ABSOLUTE 0</literal> positions before
          the first row.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>RELATIVE <replaceable class="PARAMETER">count</replaceable></literal></term>
        <listitem>
         <para>
          Fetch the <replaceable
          class="PARAMETER">count</replaceable>'th succeeding row, or
          the <literal>abs(<replaceable
          class="PARAMETER">count</replaceable>)</literal>'th prior
          row if <replaceable class="PARAMETER">count</replaceable> is
          negative.  <literal>RELATIVE 0</literal> re-fetches the
          current row, if any.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><replaceable class="PARAMETER">count</replaceable></term>
        <listitem>
         <para>
          Fetch the next <replaceable
          class="PARAMETER">count</replaceable> rows (same as
          <literal>FORWARD <replaceable
          class="PARAMETER">count</replaceable></literal>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>ALL</literal></term>
        <listitem>
         <para>
          Fetch all remaining rows (same as <literal>FORWARD ALL</literal>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>FORWARD</literal></term>
        <listitem>
         <para>
          Fetch the next row (same as <literal>NEXT</literal>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>FORWARD <replaceable class="PARAMETER">count</replaceable></literal></term>
        <listitem>
         <para>
          Fetch the next <replaceable
          class="PARAMETER">count</replaceable> rows.
          <literal>FORWARD 0</literal> re-fetches the current row.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>FORWARD ALL</literal></term>
        <listitem>
         <para>
          Fetch all remaining rows.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>BACKWARD</literal></term>
        <listitem>
         <para>
          Fetch the prior row (same as <literal>PRIOR</literal>).
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>BACKWARD <replaceable class="PARAMETER">count</replaceable></literal></term>
        <listitem>
         <para>
          Fetch the prior <replaceable
          class="PARAMETER">count</replaceable> rows (scanning
          backwards).  <literal>BACKWARD 0</literal> re-fetches the
          current row.
         </para>
        </listitem>
       </varlistentry>

       <varlistentry>
        <term><literal>BACKWARD ALL</literal></term>
        <listitem>
         <para>
          Fetch all prior rows (scanning backwards).
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">count</replaceable></term>
    <listitem>
     <para>
      <replaceable class="PARAMETER">count</replaceable> is a
      possibly-signed integer constant, determining the location or
      number of rows to fetch.  For <literal>FORWARD</> and
      <literal>BACKWARD</> cases, specifying a negative <replaceable
      class="PARAMETER">count</replaceable> is equivalent to changing
      the sense of <literal>FORWARD</> and <literal>BACKWARD</>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">cursorname</replaceable></term>
    <listitem>
     <para>
      An open cursor's name.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Outputs</title>

  <para>
   On successful completion, a <command>FETCH</> command returns a command
   tag of the form
<screen>
FETCH <replaceable class="parameter">count</replaceable>
</screen>
   The <replaceable class="parameter">count</replaceable> is the number
   of rows fetched (possibly zero).  Note that in
   <application>psql</application>, the command tag will not actually be
   displayed, since <application>psql</application> displays the fetched
   rows instead.
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   The cursor should be declared with the <literal>SCROLL</literal>
   option if one intends to use any variants of <command>FETCH</>
   other than <command>FETCH NEXT</> or <command>FETCH FORWARD</> with
   a positive count.  For simple queries
   <productname>PostgreSQL</productname> will allow backwards fetch
   from cursors not declared with <literal>SCROLL</literal>, but this
   behavior is best not relied on. If the cursor is declared with
   <literal>NO SCROLL</literal>, no backward fetches are allowed.
  </para>

  <para>
   <literal>ABSOLUTE</literal> fetches are not any faster than
   navigating to the desired row with a relative move: the underlying
   implementation must traverse all the intermediate rows anyway.
   Negative absolute fetches are even worse: the query must be read to
   the end to find the last row, and then traversed backward from
   there.  However, rewinding to the start of the query (as with
   <literal>FETCH ABSOLUTE 0</literal>) is fast.
  </para>

  <para>
   Updating data via a cursor is currently not supported by
   <productname>PostgreSQL</productname>.
  </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.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

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

<programlisting>
BEGIN WORK;

-- Set up a cursor:
DECLARE liahona SCROLL CURSOR FOR SELECT * FROM films;

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

 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

-- Fetch the previous row:
FETCH PRIOR FROM liahona;

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

-- Close the cursor and end the transaction:
CLOSE liahona;
COMMIT WORK;
</programlisting>
  </para>        
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The SQL standard defines <command>FETCH</command> for use in
   embedded SQL only.  The variant of <command>FETCH</command>
   described here returns the data as if it were a
   <command>SELECT</command> result rather than placing it in host
   variables.  Other than this point, <command>FETCH</command> is
   fully upward-compatible with the SQL standard.
  </para>

  <para>
   The <command>FETCH</command> forms involving
   <literal>FORWARD</literal> and <literal>BACKWARD</literal>, as well
   as the forms <literal>FETCH <replaceable
   class="PARAMETER">count</replaceable></literal> and <literal>FETCH
   ALL</literal>, in which <literal>FORWARD</literal> is implicit, are
   <productname>PostgreSQL</productname> extensions.
  </para>

  <para>
   The SQL standard allows only <literal>FROM</> preceding the cursor
   name; the option to use <literal>IN</> is an extension.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-close" endterm="sql-close-title"></member>
   <member><xref linkend="sql-declare" endterm="sql-declare-title"></member>
   <member><xref linkend="sql-move" endterm="sql-move-title"></member>
  </simplelist>
 </refsect1>
</refentry>

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