doc/src/sgml/ref/fetch.sgml

   1 <!--
   2 doc/src/sgml/ref/fetch.sgml
   3 PostgreSQL documentation
   4 -->
   5
   6 <refentry id="sql-fetch">
   7
   8  <indexterm zone="sql-fetch">
   9   <primary>FETCH</primary>
  10  </indexterm>
  11
  12  <indexterm zone="sql-fetch">
  13   <primary>cursor</primary>
  14   <secondary>FETCH</secondary>
  15  </indexterm>
  16  <refmeta>
  17   <refentrytitle>FETCH</refentrytitle>
  18   <manvolnum>7</manvolnum>
  19   <refmiscinfo>SQL - Language Statements</refmiscinfo>
  20  </refmeta>
  21
  22  <refnamediv>
  23   <refname>FETCH</refname>
  24   <refpurpose>retrieve rows from a query using a cursor</refpurpose>
  25  </refnamediv>
  26
  27  <refsynopsisdiv>
  28 <!-- Note the "direction" bit is also in ref/move.sgml -->
  29 <synopsis>
  30 FETCH [ <replaceable class="parameter">direction</replaceable> ] [ FROM | IN ] <replaceable class="parameter">cursor_name</replaceable>
  31
  32 <phrase>where <replaceable class="parameter">direction</replaceable> can be one of:</phrase>
  33
  34     NEXT
  35     PRIOR
  36     FIRST
  37     LAST
  38     ABSOLUTE <replaceable class="parameter">count</replaceable>
  39     RELATIVE <replaceable class="parameter">count</replaceable>
  40     <replaceable class="parameter">count</replaceable>
  41     ALL
  42     FORWARD
  43     FORWARD <replaceable class="parameter">count</replaceable>
  44     FORWARD ALL
  45     BACKWARD
  46     BACKWARD <replaceable class="parameter">count</replaceable>
  47     BACKWARD ALL
  48 </synopsis>
  49  </refsynopsisdiv>
  50
  51  <refsect1>
  52   <title>Description</title>
  53
  54   <para>
  55    <command>FETCH</command> retrieves rows using a previously-created cursor.
  56   </para>
  57
  58   <para>
  59    A cursor has an associated position, which is used by
  60    <command>FETCH</command>.  The cursor position can be before the first row of the
  61    query result, on any particular row of the result, or after the last row
  62    of the result.  When created, a cursor is positioned before the first row.
  63    After fetching some rows, the cursor is positioned on the row most recently
  64    retrieved.  If <command>FETCH</command> runs off the end of the available rows
  65    then the cursor is left positioned after the last row, or before the first
  66    row if fetching backward.  <command>FETCH ALL</command> or <command>FETCH BACKWARD
  67    ALL</command> will always leave the cursor positioned after the last row or before
  68    the first row.
  69   </para>
  70
  71   <para>
  72    The forms <literal>NEXT</literal>, <literal>PRIOR</literal>, <literal>FIRST</literal>,
  73    <literal>LAST</literal>, <literal>ABSOLUTE</literal>, <literal>RELATIVE</literal> fetch
  74    a single row after moving the cursor appropriately.  If there is no
  75    such row, an empty result is returned, and the cursor is left
  76    positioned before the first row or after the last row as
  77    appropriate.
  78   </para>
  79
  80   <para>
  81    The forms using <literal>FORWARD</literal> and <literal>BACKWARD</literal>
  82    retrieve the indicated number of rows moving in the forward or
  83    backward direction, leaving the cursor positioned on the
  84    last-returned row (or after/before all rows, if the <replaceable
  85    class="parameter">count</replaceable> exceeds the number of rows
  86    available).
  87   </para>
  88
  89   <para>
  90    <literal>RELATIVE 0</literal>, <literal>FORWARD 0</literal>, and
  91    <literal>BACKWARD 0</literal> all request fetching the current row without
  92    moving the cursor, that is, re-fetching the most recently fetched
  93    row.  This will succeed unless the cursor is positioned before the
  94    first row or after the last row; in which case, no row is returned.
  95   </para>
  96
  97   <note>
  98    <para>
  99     This page describes usage of cursors at the SQL command level.
 100     If you are trying to use cursors inside a <application>PL/pgSQL</application>
 101     function, the rules are different &mdash;
 102     see <xref linkend="plpgsql-cursor-using"/>.
 103    </para>
 104   </note>
 105  </refsect1>
 106
 107  <refsect1>
 108   <title>Parameters</title>
 109
 110   <variablelist>
 111    <varlistentry>
 112     <term><replaceable class="parameter">direction</replaceable></term>
 113     <listitem>
 114      <para><replaceable class="parameter">direction</replaceable> defines
 115       the fetch direction and number of rows to fetch.  It can be one
 116       of the following:
 117
 118       <variablelist>
 119        <varlistentry>
 120         <term><literal>NEXT</literal></term>
 121         <listitem>
 122          <para>
 123           Fetch the next row. This is the default if <replaceable
 124           class="parameter">direction</replaceable> is omitted.
 125          </para>
 126         </listitem>
 127        </varlistentry>
 128
 129        <varlistentry>
 130         <term><literal>PRIOR</literal></term>
 131         <listitem>
 132          <para>
 133           Fetch the prior row.
 134          </para>
 135         </listitem>
 136        </varlistentry>
 137
 138        <varlistentry>
 139         <term><literal>FIRST</literal></term>
 140         <listitem>
 141          <para>
 142           Fetch the first row of the query (same as <literal>ABSOLUTE 1</literal>).
 143          </para>
 144         </listitem>
 145        </varlistentry>
 146
 147        <varlistentry>
 148         <term><literal>LAST</literal></term>
 149         <listitem>
 150          <para>
 151           Fetch the last row of the query (same as <literal>ABSOLUTE -1</literal>).
 152          </para>
 153         </listitem>
 154        </varlistentry>
 155
 156        <varlistentry>
 157         <term><literal>ABSOLUTE <replaceable class="parameter">count</replaceable></literal></term>
 158         <listitem>
 159          <para>
 160           Fetch the <replaceable
 161           class="parameter">count</replaceable>'th row of the query,
 162           or the <literal>abs(<replaceable
 163           class="parameter">count</replaceable>)</literal>'th row from
 164           the end if <replaceable
 165           class="parameter">count</replaceable> is negative.  Position
 166           before first row or after last row if <replaceable
 167           class="parameter">count</replaceable> is out of range; in
 168           particular, <literal>ABSOLUTE 0</literal> positions before
 169           the first row.
 170          </para>
 171         </listitem>
 172        </varlistentry>
 173
 174        <varlistentry>
 175         <term><literal>RELATIVE <replaceable class="parameter">count</replaceable></literal></term>
 176         <listitem>
 177          <para>
 178           Fetch the <replaceable
 179           class="parameter">count</replaceable>'th succeeding row, or
 180           the <literal>abs(<replaceable
 181           class="parameter">count</replaceable>)</literal>'th prior
 182           row if <replaceable class="parameter">count</replaceable> is
 183           negative.  <literal>RELATIVE 0</literal> re-fetches the
 184           current row, if any.
 185          </para>
 186         </listitem>
 187        </varlistentry>
 188
 189        <varlistentry>
 190         <term><replaceable class="parameter">count</replaceable></term>
 191         <listitem>
 192          <para>
 193           Fetch the next <replaceable
 194           class="parameter">count</replaceable> rows (same as
 195           <literal>FORWARD <replaceable
 196           class="parameter">count</replaceable></literal>).
 197          </para>
 198         </listitem>
 199        </varlistentry>
 200
 201        <varlistentry>
 202         <term><literal>ALL</literal></term>
 203         <listitem>
 204          <para>
 205           Fetch all remaining rows (same as <literal>FORWARD ALL</literal>).
 206          </para>
 207         </listitem>
 208        </varlistentry>
 209
 210        <varlistentry>
 211         <term><literal>FORWARD</literal></term>
 212         <listitem>
 213          <para>
 214           Fetch the next row (same as <literal>NEXT</literal>).
 215          </para>
 216         </listitem>
 217        </varlistentry>
 218
 219        <varlistentry>
 220         <term><literal>FORWARD <replaceable class="parameter">count</replaceable></literal></term>
 221         <listitem>
 222          <para>
 223           Fetch the next <replaceable
 224           class="parameter">count</replaceable> rows.
 225           <literal>FORWARD 0</literal> re-fetches the current row.
 226          </para>
 227         </listitem>
 228        </varlistentry>
 229
 230        <varlistentry>
 231         <term><literal>FORWARD ALL</literal></term>
 232         <listitem>
 233          <para>
 234           Fetch all remaining rows.
 235          </para>
 236         </listitem>
 237        </varlistentry>
 238
 239        <varlistentry>
 240         <term><literal>BACKWARD</literal></term>
 241         <listitem>
 242          <para>
 243           Fetch the prior row (same as <literal>PRIOR</literal>).
 244          </para>
 245         </listitem>
 246        </varlistentry>
 247
 248        <varlistentry>
 249         <term><literal>BACKWARD <replaceable class="parameter">count</replaceable></literal></term>
 250         <listitem>
 251          <para>
 252           Fetch the prior <replaceable
 253           class="parameter">count</replaceable> rows (scanning
 254           backwards).  <literal>BACKWARD 0</literal> re-fetches the
 255           current row.
 256          </para>
 257         </listitem>
 258        </varlistentry>
 259
 260        <varlistentry>
 261         <term><literal>BACKWARD ALL</literal></term>
 262         <listitem>
 263          <para>
 264           Fetch all prior rows (scanning backwards).
 265          </para>
 266         </listitem>
 267        </varlistentry>
 268       </variablelist></para>
 269     </listitem>
 270    </varlistentry>
 271
 272    <varlistentry>
 273     <term><replaceable class="parameter">count</replaceable></term>
 274     <listitem>
 275      <para><replaceable class="parameter">count</replaceable> is a
 276       possibly-signed integer constant, determining the location or
 277       number of rows to fetch.  For <literal>FORWARD</literal> and
 278       <literal>BACKWARD</literal> cases, specifying a negative <replaceable
 279       class="parameter">count</replaceable> is equivalent to changing
 280       the sense of <literal>FORWARD</literal> and <literal>BACKWARD</literal>.
 281      </para>
 282     </listitem>
 283    </varlistentry>
 284
 285    <varlistentry>
 286     <term><replaceable class="parameter">cursor_name</replaceable></term>
 287     <listitem>
 288      <para>
 289       An open cursor's name.
 290      </para>
 291     </listitem>
 292    </varlistentry>
 293   </variablelist>
 294  </refsect1>
 295
 296  <refsect1>
 297   <title>Outputs</title>
 298
 299   <para>
 300    On successful completion, a <command>FETCH</command> command returns a command
 301    tag of the form
 302 <screen>
 303 FETCH <replaceable class="parameter">count</replaceable>
 304 </screen>
 305    The <replaceable class="parameter">count</replaceable> is the number
 306    of rows fetched (possibly zero).  Note that in
 307    <application>psql</application>, the command tag will not actually be
 308    displayed, since <application>psql</application> displays the fetched
 309    rows instead.
 310   </para>
 311  </refsect1>
 312
 313  <refsect1>
 314   <title>Notes</title>
 315
 316   <para>
 317    The cursor should be declared with the <literal>SCROLL</literal>
 318    option if one intends to use any variants of <command>FETCH</command>
 319    other than <command>FETCH NEXT</command> or <command>FETCH FORWARD</command> with
 320    a positive count.  For simple queries
 321    <productname>PostgreSQL</productname> will allow backwards fetch
 322    from cursors not declared with <literal>SCROLL</literal>, but this
 323    behavior is best not relied on. If the cursor is declared with
 324    <literal>NO SCROLL</literal>, no backward fetches are allowed.
 325   </para>
 326
 327   <para>
 328    <literal>ABSOLUTE</literal> fetches are not any faster than
 329    navigating to the desired row with a relative move: the underlying
 330    implementation must traverse all the intermediate rows anyway.
 331    Negative absolute fetches are even worse: the query must be read to
 332    the end to find the last row, and then traversed backward from
 333    there.  However, rewinding to the start of the query (as with
 334    <literal>FETCH ABSOLUTE 0</literal>) is fast.
 335   </para>
 336
 337   <para>
 338    <link linkend="sql-declare"><command>DECLARE</command></link>
 339    is used to define a cursor.  Use
 340    <link linkend="sql-move"><command>MOVE</command></link>
 341    to change cursor position without retrieving data.
 342   </para>
 343  </refsect1>
 344
 345  <refsect1>
 346   <title>Examples</title>
 347
 348   <para>
 349    The following example traverses a table using a cursor:
 350
 351 <programlisting>
 352 BEGIN WORK;
 353
 354 -- Set up a cursor:
 355 DECLARE liahona SCROLL CURSOR FOR SELECT * FROM films;
 356
 357 -- Fetch the first 5 rows in the cursor liahona:
 358 FETCH FORWARD 5 FROM liahona;
 359
 360  code  |          title          | did | date_prod  |   kind   |  len
 361 -------+-------------------------+-----+------------+----------+-------
 362  BL101 | The Third Man           | 101 | 1949-12-23 | Drama    | 01:44
 363  BL102 | The African Queen       | 101 | 1951-08-11 | Romantic | 01:43
 364  JL201 | Une Femme est une Femme | 102 | 1961-03-12 | Romantic | 01:25
 365  P_301 | Vertigo                 | 103 | 1958-11-14 | Action   | 02:08
 366  P_302 | Becket                  | 103 | 1964-02-03 | Drama    | 02:28
 367
 368 -- Fetch the previous row:
 369 FETCH PRIOR FROM liahona;
 370
 371  code  |  title  | did | date_prod  |  kind  |  len
 372 -------+---------+-----+------------+--------+-------
 373  P_301 | Vertigo | 103 | 1958-11-14 | Action | 02:08
 374
 375 -- Close the cursor and end the transaction:
 376 CLOSE liahona;
 377 COMMIT WORK;
 378 </programlisting></para>
 379  </refsect1>
 380
 381  <refsect1>
 382   <title>Compatibility</title>
 383
 384   <para>
 385    The SQL standard defines <command>FETCH</command> for use in
 386    embedded SQL only.  The variant of <command>FETCH</command>
 387    described here returns the data as if it were a
 388    <command>SELECT</command> result rather than placing it in host
 389    variables.  Other than this point, <command>FETCH</command> is
 390    fully upward-compatible with the SQL standard.
 391   </para>
 392
 393   <para>
 394    The <command>FETCH</command> forms involving
 395    <literal>FORWARD</literal> and <literal>BACKWARD</literal>, as well
 396    as the forms <literal>FETCH <replaceable
 397    class="parameter">count</replaceable></literal> and <literal>FETCH
 398    ALL</literal>, in which <literal>FORWARD</literal> is implicit, are
 399    <productname>PostgreSQL</productname> extensions.
 400   </para>
 401
 402   <para>
 403    The SQL standard allows only <literal>FROM</literal> preceding the cursor
 404    name; the option to use <literal>IN</literal>, or to leave them out altogether, is
 405    an extension.
 406   </para>
 407  </refsect1>
 408
 409  <refsect1>
 410   <title>See Also</title>
 411
 412   <simplelist type="inline">
 413    <member><xref linkend="sql-close"/></member>
 414    <member><xref linkend="sql-declare"/></member>
 415    <member><xref linkend="sql-move"/></member>
 416   </simplelist>
 417  </refsect1>
 418 </refentry>