doc/src/sgml/ref/values.sgml - master - githubmirror/postgres

doc/src/sgml/ref/values.sgml
251 строка8 KB
Peter Eisentraut
doc: Properly punctuate "etc."
19 май 2022, 10:42
19 май 2022, 10:42648aa67
<!--
doc/src/sgml/ref/values.sgml
PostgreSQL documentation
-->

<refentry id="sql-values">
 <indexterm zone="sql-values">
  <primary>VALUES</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>VALUES</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>VALUES</refname>
  <refpurpose>compute a set of rows</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
VALUES ( <replaceable class="parameter">expression</replaceable> [, ...] ) [, ...]
    [ ORDER BY <replaceable class="parameter">sort_expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [, ...] ]
    [ LIMIT { <replaceable class="parameter">count</replaceable> | ALL } ]
    [ OFFSET <replaceable class="parameter">start</replaceable> [ ROW | ROWS ] ]
    [ FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } ONLY ]
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>VALUES</command> computes a row value or set of row values
   specified by value expressions.  It is most commonly used to generate
   a <quote>constant table</quote> within a larger command, but it can be
   used on its own.
  </para>

  <para>
   When more than one row is specified, all the rows must have the same
   number of elements.  The data types of the resulting table's columns are
   determined by combining the explicit or inferred types of the expressions
   appearing in that column, using the same rules as for <literal>UNION</literal>
   (see <xref linkend="typeconv-union-case"/>).
  </para>

  <para>
   Within larger commands, <command>VALUES</command> is syntactically allowed
   anywhere that <command>SELECT</command> is.  Because it is treated like a
   <command>SELECT</command> by the grammar, it is possible to use
   the <literal>ORDER BY</literal>, <literal>LIMIT</literal> (or
   equivalently <literal>FETCH FIRST</literal>),
   and <literal>OFFSET</literal> clauses with a
   <command>VALUES</command> command.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">expression</replaceable></term>
    <listitem>
     <para>
      A constant or expression to compute and insert at the indicated place
      in the resulting table (set of rows).  In a <command>VALUES</command> list
      appearing at the top level of an <command>INSERT</command>, an
      <replaceable class="parameter">expression</replaceable> can be replaced
      by <literal>DEFAULT</literal> to indicate that the destination column's
      default value should be inserted.  <literal>DEFAULT</literal> cannot
      be used when <command>VALUES</command> appears in other contexts.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">sort_expression</replaceable></term>
    <listitem>
     <para>
      An expression or integer constant indicating how to sort the result
      rows.  This expression can refer to the columns of the
      <command>VALUES</command> result as <literal>column1</literal>, <literal>column2</literal>,
      etc.  For more details see
      <xref linkend="sql-orderby"/>
      in the <xref linkend="sql-select"/> documentation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">operator</replaceable></term>
    <listitem>
     <para>
      A sorting operator.  For details see
      <xref linkend="sql-orderby"/>
      in the <xref linkend="sql-select"/> documentation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">count</replaceable></term>
    <listitem>
     <para>
      The maximum number of rows to return.  For details see
      <xref linkend="sql-limit"/>
      in the <xref linkend="sql-select"/> documentation.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">start</replaceable></term>
    <listitem>
     <para>
      The number of rows to skip before starting to return rows.
      For details see <xref linkend="sql-limit"/>
      in the <xref linkend="sql-select"/> documentation.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   <command>VALUES</command> lists with very large numbers of rows should be avoided,
   as you might encounter out-of-memory failures or poor performance.
   <command>VALUES</command> appearing within <command>INSERT</command> is a special case
   (because the desired column types are known from the <command>INSERT</command>'s
   target table, and need not be inferred by scanning the <command>VALUES</command>
   list), so it can handle larger lists than are practical in other contexts.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   A bare <command>VALUES</command> command:

<programlisting>
VALUES (1, 'one'), (2, 'two'), (3, 'three');
</programlisting>

   This will return a table of two columns and three rows.  It's effectively
   equivalent to:

<programlisting>
SELECT 1 AS column1, 'one' AS column2
UNION ALL
SELECT 2, 'two'
UNION ALL
SELECT 3, 'three';
</programlisting>

  </para>

  <para>
   More usually, <command>VALUES</command> is used within a larger SQL command.
   The most common use is in <command>INSERT</command>:

<programlisting>
INSERT INTO films (code, title, did, date_prod, kind)
    VALUES ('T_601', 'Yojimbo', 106, '1961-06-16', 'Drama');
</programlisting>
  </para>

  <para>
   In the context of <command>INSERT</command>, entries of a <command>VALUES</command> list
   can be <literal>DEFAULT</literal> to indicate that the column default
   should be used here instead of specifying a value:

<programlisting>
INSERT INTO films VALUES
    ('UA502', 'Bananas', 105, DEFAULT, 'Comedy', '82 minutes'),
    ('T_601', 'Yojimbo', 106, DEFAULT, 'Drama', DEFAULT);
</programlisting>
  </para>

  <para>
   <command>VALUES</command> can also be used where a sub-<command>SELECT</command> might
   be written, for example in a <literal>FROM</literal> clause:

<programlisting>
SELECT f.*
  FROM films f, (VALUES('MGM', 'Horror'), ('UA', 'Sci-Fi')) AS t (studio, kind)
  WHERE f.studio = t.studio AND f.kind = t.kind;

UPDATE employees SET salary = salary * v.increase
  FROM (VALUES(1, 200000, 1.2), (2, 400000, 1.4)) AS v (depno, target, increase)
  WHERE employees.depno = v.depno AND employees.sales &gt;= v.target;
</programlisting>

   Note that an <literal>AS</literal> clause is required when <command>VALUES</command>
   is used in a <literal>FROM</literal> clause, just as is true for
   <command>SELECT</command>.  It is not required that the <literal>AS</literal> clause
   specify names for all the columns, but it's good practice to do so.
   (The default column names for <command>VALUES</command> are <literal>column1</literal>,
   <literal>column2</literal>, etc. in <productname>PostgreSQL</productname>, but
   these names might be different in other database systems.)
  </para>

  <para>
   When <command>VALUES</command> is used in <command>INSERT</command>, the values are all
   automatically coerced to the data type of the corresponding destination
   column.  When it's used in other contexts, it might be necessary to specify
   the correct data type.  If the entries are all quoted literal constants,
   coercing the first is sufficient to determine the assumed type for all:

<programlisting>
SELECT * FROM machines
WHERE ip_address IN (VALUES('192.168.0.1'::inet), ('192.168.0.10'), ('192.168.1.43'));
</programlisting></para>

  <tip>
   <para>
    For simple <literal>IN</literal> tests, it's better to rely on the
    <link linkend="functions-comparisons-in-scalar">list-of-scalars</link>
    form of <literal>IN</literal> than to write a <command>VALUES</command>
    query as shown above.  The list of scalars method requires less writing
    and is often more efficient.
   </para>
  </tip>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para><command>VALUES</command> conforms to the SQL standard.
   <literal>LIMIT</literal> and <literal>OFFSET</literal> are
   <productname>PostgreSQL</productname> extensions; see also
   under <xref linkend="sql-select"/>.
  </para>
 </refsect1>

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

  <simplelist type="inline">
   <member><xref linkend="sql-insert"/></member>
   <member><xref linkend="sql-select"/></member>
  </simplelist>
 </refsect1>
</refentry>