path: root/doc/src/sgml/ref/select.sgml

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Postgres documentation
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<refentry id="SQL-SELECT">
 <refmeta>
  <refentrytitle id="sql-select-title">
   SELECT
  </refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   SELECT
  </refname>
  <refpurpose>
   Retrieve rows from a table or view.
  </refpurpose></refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ]
    <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
    [ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
    [ FROM <replaceable class="PARAMETER">table</replaceable> [ <replaceable class="PARAMETER">alias</replaceable> ] [, ...] ]
    [ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
    [ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
    [ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
    [ { UNION [ ALL ] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable> ]
    [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
    [ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
    LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
  </synopsis>
  
  <refsect2 id="R2-SQL-SELECT-1">
   <refsect2info>
    <date>2000-03-15</date>
   </refsect2info>
   <title>
    Inputs
   </title>

   <para>
    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER">expression</replaceable></term>
      <listitem>
       <para>
	The name of a table's column or an expression.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">name</replaceable></term>
      <listitem>
       <para>
	Specifies another name for a column or an expression using
	the AS clause.  This name is primarily used to label the column
	for display.  It can also be used to refer to the column's value in
	ORDER BY and GROUP BY clauses.  But the
	<replaceable class="PARAMETER">name</replaceable>
	cannot be used in the WHERE or HAVING clauses; write out the
	expression instead.
       </para>
      </listitem>
     </varlistentry>
     
    <varlistentry>
     <term>TEMPORARY</term>
     <term>TEMP</term>
     <listitem>
      <para>
	If TEMPORARY or TEMP is specified,
	the table is created unique to this session, and is
	automatically dropped on session exit.
      </para>
     </listitem>
    </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">new_table</replaceable></term>
      <listitem>
       <para>
	If the INTO TABLE clause is specified, the result of the
	query will be stored in a new table with the indicated
	name.
	The target table (<replaceable class="PARAMETER">new_table</replaceable>) will
	be created automatically and must not exist before this command.
        Refer to <command>SELECT INTO</command> for more information.

	<note>
	 <para>
	  The <command>CREATE TABLE AS</command> statement will also
	  create a new  table from a select query.
	 </para>
	</note>
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">table</replaceable></term>
      <listitem>
       <para>
	The name of an existing table referenced by the FROM clause.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">alias</replaceable></term>
      <listitem>
       <para>
	An alternate name for the preceding
	<replaceable class="PARAMETER">table</replaceable>.
	It is used for brevity or to eliminate ambiguity for joins
	within a single table.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">condition</replaceable></term>
      <listitem>
       <para>
	A boolean expression giving a result of true or false.
	See the WHERE clause.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">column</replaceable></term>
      <listitem>
       <para>
	The name of a table's column.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">select</replaceable></term>
      <listitem>
       <para>
	A select statement with all features except the ORDER BY and
	LIMIT clauses.
       </para>
      </listitem>
     </varlistentry>
     
    </variablelist>
   </para>
  </refsect2>
  
  <refsect2 id="R2-SQL-SELECT-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term>Rows</term>
      <listitem>
       <para>
	The complete set of rows resulting from the query specification.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       <returnvalue><replaceable>count</replaceable></returnvalue>
      </term>
      <listitem>
       <para>
	The count of rows returned by the query.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-SELECT-1">
  <refsect1info>
   <date>2000-03-15</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>SELECT</command> will return rows from one or more tables.
   Candidates for selection are rows which satisfy the WHERE condition;
   if WHERE is omitted, all rows are candidates.
   (See <xref linkend="sql-where" endterm="sql-where-title">.)
  </para>

  <para>
   <command>DISTINCT</command> will eliminate duplicate rows from the
   result.
   <command>ALL</command> (the default) will return all candidate rows,
   including duplicates.
  </para>

  <para>
   <command>DISTINCT ON</command> eliminates rows that match on all the
   specified expressions, keeping only the first row of each set of
   duplicates.  The DISTINCT ON expressions are interpreted using the
   same rules as for ORDER BY items; see below.
   Note that "the first row" of each set is unpredictable
   unless <command>ORDER BY</command> is used to ensure that the desired
   row appears first.  For example,
   <programlisting>
        SELECT DISTINCT ON (location) location, time, report
        FROM weatherReports
        ORDER BY location, time DESC;
   </programlisting>
   retrieves the most recent weather report for each location.  But if
   we had not used ORDER BY to force descending order of time values
   for each location, we'd have gotten a report of unpredictable age
   for each location.
  </para>

  <para>
   The GROUP BY clause allows a user to divide a table
   into groups of rows that match on one or more values.
   (See <xref linkend="sql-groupby" endterm="sql-groupby-title">.)
  </para>

  <para>
   The HAVING clause allows selection of only those groups of rows
   meeting the specified condition.
   (See <xref linkend="sql-having" endterm="sql-having-title">.)
  </para>
   
  <para>
   The ORDER BY clause causes the returned rows to be sorted in a specified
   order.  If ORDER BY is not given, the rows are returned in whatever order
   the system finds cheapest to produce.
   (See <xref linkend="sql-orderby-title" endterm="sql-orderby-title">.)
  </para>
   
  <para>
   The UNION operator allows the result to be the collection of rows
   returned by the queries involved.
   (See <xref linkend="sql-union" endterm="sql-union-title">.)
  </para>
   
  <para>
   The INTERSECT operator gives you the rows that are common to both queries.
   (See <xref linkend="sql-intersect" endterm="sql-intersect-title">.)
  </para>
   
  <para>
   The EXCEPT operator gives you the rows returned by the first query but
   not the second query.
   (See <xref linkend="sql-except" endterm="sql-except-title">.)
  </para>
   
  <para>
   The FOR UPDATE clause allows the SELECT statement to perform 
   exclusive locking of selected rows.
  </para>
   
  <para>
   The LIMIT clause allows a subset of the rows produced by the query
   to be returned to the user.
   (See <xref linkend="sql-limit" endterm="sql-limit-title">.)
  </para>

  <para>
   You must have SELECT privilege to a table to read its values
   (See the <command>GRANT</command>/<command>REVOKE</command> statements).
  </para>
   
  <refsect2 id="SQL-WHERE">
   <refsect2info>
    <date>2000-03-15</date>
   </refsect2info>
   <title id="sql-where-title">
    WHERE Clause
   </title>

   <para>
    The optional WHERE condition has the general form:
    
    <synopsis>
WHERE <replaceable class="PARAMETER">boolean_expr</replaceable>
    </synopsis>
    
    <replaceable class="PARAMETER">boolean_expr</replaceable>
    can consist of any expression which evaluates to a boolean value.
    In many cases, this expression will be

    <synopsis>
     <replaceable class="PARAMETER">expr</replaceable> <replaceable class="PARAMETER">cond_op</replaceable> <replaceable class="PARAMETER">expr</replaceable>
    </synopsis>

    or

    <synopsis>
     <replaceable class="PARAMETER">log_op</replaceable> <replaceable class="PARAMETER">expr</replaceable>
    </synopsis>

    where <replaceable class="PARAMETER">cond_op</replaceable>
    can be one of: =, &lt;, &lt;=, &gt;, &gt;= or &lt;&gt;,
    a conditional operator like ALL, ANY, IN, LIKE, or a
    locally-defined operator, 
    and <replaceable class="PARAMETER">log_op</replaceable> can be one 
    of: AND, OR, NOT.
    SELECT will ignore all rows for which the WHERE condition does not return
    TRUE.
   </para>
  </refsect2>
  
  <refsect2 id="SQL-GROUPBY">
   <refsect2info>
    <date>2000-03-15</date>
   </refsect2info>
   <title id="sql-groupby-title">
    GROUP BY Clause
   </title>
   <para>
    GROUP BY specifies a grouped table derived by the application
    of this clause:
    <synopsis>
GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...]
    </synopsis>
   </para>

   <para>
    GROUP BY will condense into a single row all selected rows that share the
    same values for the grouped columns.  Aggregate functions, if any,
    are computed across all rows making up each group, producing a
    separate value for each group (whereas without GROUP BY, an
    aggregate produces a single value computed across all the selected
    rows).  When GROUP BY is present, it is not valid for the SELECT
    output expression(s) to refer to
    ungrouped columns except within aggregate functions, since there
    would be more than one possible value to return for an ungrouped column.
   </para>

   <para>
    An item in GROUP BY can also be the name or ordinal number of an output
    column (SELECT expression), or it can be an arbitrary expression formed
    from input-column values.  In case of ambiguity, a GROUP BY name will
    be interpreted as an input-column name rather than an output column name.
   </para>
  </refsect2>

  <refsect2 id="SQL-HAVING">
   <refsect2info>
    <date>2000-03-15</date>
   </refsect2info>
   <title id="sql-having-title">
    HAVING Clause
   </title>
   <para>
    The optional HAVING condition has the general form:
    
    <synopsis>
HAVING <replaceable class="PARAMETER">cond_expr</replaceable>
    </synopsis>
    
    where <replaceable class="PARAMETER">cond_expr</replaceable> is the same
    as specified for the WHERE clause.
   </para>
    
   <para>
    HAVING specifies a grouped table derived by the elimination
    of group rows that do not satisfy the
    <replaceable class="PARAMETER">cond_expr</replaceable>.
    HAVING is different from WHERE:
    WHERE filters individual rows before application of GROUP BY,
    while HAVING filters group rows created by GROUP BY.
   </para>

   <para>
    Each column referenced in 
    <replaceable class="PARAMETER">cond_expr</replaceable> shall unambiguously
    reference a grouping column, unless the reference appears within an
    aggregate function.
   </para>
  </refsect2>
  
  <refsect2 id="SQL-ORDERBY">
   <refsect2info>
    <date>2000-03-15</date>
   </refsect2info>
   <title id="sql-orderby-title">
    ORDER BY Clause
   </title>
   <para>
    <synopsis>
ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...]
    </synopsis></para>
    
   <para>
    <replaceable class="PARAMETER">column</replaceable> can be either a
    result column name or an ordinal number.
   </para>
   <para>
    The ordinal numbers refers to the ordinal (left-to-right) position
    of the result column. This feature makes it possible to define an ordering
    on the basis of a column that does not have a proper name.
    This is never absolutely necessary because it is always possible
    to assign a name to a result column using the AS clause, e.g.:
    <programlisting>
SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
    </programlisting></para>
    
   <para>
    It is also possible to ORDER BY
    arbitrary expressions (an extension to SQL92),
    including fields that do not appear in the
    SELECT result list.
    Thus the following statement is legal:
    <programlisting>
SELECT name FROM distributors ORDER BY code;
    </programlisting>

    Note that if an ORDER BY item is a simple name that matches both
    a result column name and an input column name, ORDER BY will interpret
    it as the result column name.  This is the opposite of the choice that
    GROUP BY will make in the same situation.  This inconsistency is
    mandated by the SQL92 standard.
   </para>
    
   <para>
    Optionally one may add the keyword DESC (descending)
    or ASC (ascending) after each column name in the ORDER BY clause.
    If not specified, ASC is assumed by default.  Alternatively, a
    specific ordering operator name may be specified.  ASC is equivalent
    to USING '&lt;' and DESC is equivalent to USING '&gt;'.
   </para>
  </refsect2>
  
  <refsect2 id="SQL-UNION">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title id="sql-union-title">
    UNION Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> UNION [ ALL ] <replaceable class="PARAMETER">table_query</replaceable>
    [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
    </synopsis>

    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY or LIMIT clause.
   </para>
    
   <para>
    The UNION operator allows the result to be the collection of rows
    returned by the queries involved.
    The two SELECTs that represent the direct operands of the UNION must
    produce the same number of columns, and corresponding columns must be
    of compatible data types.
   </para>
    
   <para>
    By default, the result of UNION does not contain any duplicate rows
    unless the ALL clause is specified.
   </para>
    
   <para>
    Multiple UNION operators in the same SELECT statement are
    evaluated left to right.
    Note that the ALL keyword is not global in nature, being 
    applied only for the current pair of table results.
   </para>

  </refsect2>

  <refsect2 id="SQL-INTERSECT">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title id="sql-intersect-title">
    INTERSECT Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> INTERSECT <replaceable class="PARAMETER">table_query</replaceable>
    [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
    </synopsis>
    
    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY or LIMIT clause.
   </para>

   <para>
    The INTERSECT operator gives you the rows that are common to both queries.
    The two SELECTs that represent the direct operands of the INTERSECT must
    produce the same number of columns, and corresponding columns must be
    of compatible data types.
   </para>
    
   <para>
    Multiple INTERSECT operators in the same SELECT statement are
    evaluated left to right, unless parentheses dictate otherwise.
   </para>
  </refsect2>

  <refsect2 id="SQL-EXCEPT">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title id="sql-except-title">
    EXCEPT Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> EXCEPT <replaceable class="PARAMETER">table_query</replaceable>
     [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
    </synopsis>
    
    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY or LIMIT clause.
   </para>

   <para>
    The EXCEPT operator gives you the rows returned by the first query but
    not the second query.
    The two SELECTs that represent the direct operands of the EXCEPT must
    produce the same number of columns, and corresponding columns must be
    of compatible data types.
   </para>

   <para>
    Multiple EXCEPT operators in the same SELECT statement are
    evaluated left to right, unless parentheses dictate otherwise.
   </para>
  </refsect2>

  <refsect2 id="SQL-LIMIT">
   <refsect2info>
    <date>2000-02-20</date>
   </refsect2info>
   <title id="sql-limit-title">
    LIMIT Clause
   </title>
   <para>
    <synopsis>
    LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
    OFFSET <replaceable class="PARAMETER">start</replaceable>
    </synopsis>
    
    where
    <replaceable class="PARAMETER">count</replaceable> specifies the
    maximum number of rows to return, and
    <replaceable class="PARAMETER">start</replaceable> specifies the
    number of rows to skip before starting to return rows.
   </para>

   <para>
    LIMIT allows you to retrieve just a portion of the rows that are generated
    by the rest of the query.  If a limit count is given, no more than that
    many rows will be returned.  If an offset is given, that many rows will
    be skipped before starting to return rows.
   </para>

   <para>
    When using LIMIT, it is a good idea to use an ORDER BY clause that
    constrains the result rows into a unique order.  Otherwise you will get
    an unpredictable subset of the query's rows --- you may be asking for
    the tenth through twentieth rows, but tenth through twentieth in what
    ordering?  You don't know what ordering, unless you specified ORDER BY.
   </para>

   <para>
    As of <productname>Postgres</productname> 7.0, the
    query optimizer takes LIMIT into account when generating a query plan,
    so you are very likely to get different plans (yielding different row
    orders) depending on what you give for LIMIT and OFFSET.  Thus, using
    different LIMIT/OFFSET values to select different subsets of a query
    result <emphasis>will give inconsistent results</emphasis> unless
    you enforce a predictable result ordering with ORDER BY.  This is not
    a bug; it is an inherent consequence of the fact that SQL does not
    promise to deliver the results of a query in any particular order
    unless ORDER BY is used to constrain the order.
   </para>
  </refsect2>
 </refsect1>

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

  <para>
   To join the table <literal>films</literal> with the table
   <literal>distributors</literal>:

   <programlisting>
SELECT f.title, f.did, d.name, f.date_prod, f.kind
    FROM distributors d, films f
    WHERE f.did = d.did

           title           | did |   name           | date_prod  | kind
---------------------------+-----+------------------+------------+----------
 The Third Man             | 101 | British Lion     | 1949-12-23 | Drama
 The African Queen         | 101 | British Lion     | 1951-08-11 | Romantic
 Une Femme est une Femme   | 102 | Jean Luc Godard  | 1961-03-12 | Romantic
 Vertigo                   | 103 | Paramount        | 1958-11-14 | Action
 Becket                    | 103 | Paramount        | 1964-02-03 | Drama
 48 Hrs                    | 103 | Paramount        | 1982-10-22 | Action
 War and Peace             | 104 | Mosfilm          | 1967-02-12 | Drama
 West Side Story           | 105 | United Artists   | 1961-01-03 | Musical
 Bananas                   | 105 | United Artists   | 1971-07-13 | Comedy
 Yojimbo                   | 106 | Toho             | 1961-06-16 | Drama
 There's a Girl in my Soup | 107 | Columbia         | 1970-06-11 | Comedy
 Taxi Driver               | 107 | Columbia         | 1975-05-15 | Action
 Absence of Malice         | 107 | Columbia         | 1981-11-15 | Action
 Storia di una donna       | 108 | Westward         | 1970-08-15 | Romantic
 The King and I            | 109 | 20th Century Fox | 1956-08-11 | Musical
 Das Boot                  | 110 | Bavaria Atelier  | 1981-11-11 | Drama
 Bed Knobs and Broomsticks | 111 | Walt Disney      |            | Musical
(17 rows)
</programlisting>
  </para>

  <para>
   To sum the column <literal>len</literal> of all films and group
   the results by <literal>kind</literal>:

<programlisting>
SELECT kind, SUM(len) AS total FROM films GROUP BY kind;

   kind   | total
----------+-------
 Action   | 07:34
 Comedy   | 02:58
 Drama    | 14:28
 Musical  | 06:42
 Romantic | 04:38
(5 rows)
</programlisting>
  </para>

  <para>
   To sum the column <literal>len</literal> of all films, group
   the results by <literal>kind</literal> and show those group totals
   that are less than 5 hours:

<programlisting>
SELECT kind, SUM(len) AS total
    FROM films
    GROUP BY kind
    HAVING SUM(len) < INTERVAL '5 hour';

 kind     | total
----------+-------
 Comedy   | 02:58
 Romantic | 04:38
(2 rows)
</programlisting>
  </para>

  <para>
   The following two examples are identical ways of sorting the individual
   results according to the contents of the second column
   (<literal>name</literal>):

   <programlisting>
SELECT * FROM distributors ORDER BY name;
SELECT * FROM distributors ORDER BY 2;

 did |       name
-----+------------------
 109 | 20th Century Fox
 110 | Bavaria Atelier
 101 | British Lion
 107 | Columbia
 102 | Jean Luc Godard
 113 | Luso films
 104 | Mosfilm
 103 | Paramount
 106 | Toho
 105 | United Artists
 111 | Walt Disney
 112 | Warner Bros.
 108 | Westward
(13 rows)
</programlisting>
  </para>

  <para>
   This example shows how to obtain the union of the tables
   <literal>distributors</literal> and
   <literal>actors</literal>, restricting the results to those that begin
   with letter W in each table.  Only distinct rows are wanted, so the
   ALL keyword is omitted:

<programlisting>
distributors:               actors:
 did |     name              id |     name
-----+--------------        ----+----------------
 108 | Westward               1 | Woody Allen
 111 | Walt Disney            2 | Warren Beatty
 112 | Warner Bros.           3 | Walter Matthau
 ...                         ...

SELECT distributors.name
    FROM   distributors
    WHERE  distributors.name LIKE 'W%'
UNION
SELECT actors.name
    FROM   actors
    WHERE  actors.name LIKE 'W%'

      name
----------------
 Walt Disney
 Walter Matthau
 Warner Bros.
 Warren Beatty
 Westward
 Woody Allen
</programlisting>
  </para>
 </refsect1>
 
 <refsect1 id="R1-SQL-SELECT-3">
  <title>
   Compatibility
  </title>
  
  <refsect2 id="R2-SQL-SELECT-4">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    <acronym>Extensions</acronym>
   </title>

   <para>
<productname>Postgres</productname> allows one to omit 
the <command>FROM</command> clause from a query. This feature
was retained from the original PostQuel query language:
  <programlisting>
SELECT distributors.* WHERE name = 'Westwood';

 did | name
-----+----------
 108 | Westward
  </programlisting>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-SELECT-5">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    <acronym>SQL92</acronym>
   </title>
   <para>
   </para>
   
   <refsect3 id="R3-SQL-SELECT-1">
    <refsect3info>
     <date>1998-04-15</date>
    </refsect3info>
    <title>
     SELECT Clause
    </title>
    <para>
     In the <acronym>SQL92</acronym> standard, the optional keyword "AS"
     is just noise and can be 
     omitted without affecting the meaning.
     The <productname>Postgres</productname> parser requires this keyword when
     renaming columns because the type extensibility features lead to
     parsing ambiguities
     in this context.</para>
     
    <para>
     The DISTINCT ON phrase is not part of <acronym>SQL92</acronym>.
     Nor are LIMIT and OFFSET.
    </para>
     
    <para>
     In <acronym>SQL92</acronym>, an ORDER BY clause may only use result
     column names or numbers, while a GROUP BY clause may only use input
     column names.
     <productname>Postgres</productname> extends each of these clauses to
     allow the other choice as well (but it uses the standard's interpretation
     if there is ambiguity).
     <productname>Postgres</productname> also allows both clauses to specify
     arbitrary expressions.  Note that names appearing in an expression will
     always be taken as input-column names, not as result-column names.
    </para>
   </refsect3>

   <refsect3 id="R3-SQL-UNION-1">
    <refsect3info>
     <date>1998-09-24</date>
    </refsect3info>
    <title>
     UNION Clause
    </title>
    <para>
     The <acronym>SQL92</acronym> syntax for UNION allows an
     additional CORRESPONDING BY clause:
     <synopsis> 
<replaceable class="PARAMETER">table_query</replaceable> UNION [ALL]
    [CORRESPONDING [BY (<replaceable class="PARAMETER">column</replaceable> [,...])]]
    <replaceable class="PARAMETER">table_query</replaceable>
     </synopsis></para>

    <para>
     The CORRESPONDING BY clause is not supported by
     <productname>Postgres</productname>.
    </para>
   </refsect3>
   
  </refsect2>
 </refsect1>
</refentry>

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