select.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.24 2000/01/27 18:11:25 tgl Exp $
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 ] [, ...] ]
    [ FOR UPDATE [ OF class_name... ] ]
    [ LIMIT { count | ALL } [ { OFFSET | , } count ] ]
  </synopsis>
  
  <refsect2 id="R2-SQL-SELECT-1">
   <refsect2info>
    <date>1998-09-24</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. <replaceable class="PARAMETER">name</replaceable>
	cannot be used in the WHERE
	condition. It can, however, be referenced in associated
	ORDER BY or GROUP BY clauses.
       </para>
      </listitem>
     </varlistentry>
     
    <varlistentry>
     <term>TEMPORARY</term>
     <term>TEMP</term>
     <listitem>
      <para>
	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 another table with the indicated
	name.
	The target table (<replaceable class="PARAMETER">new_table</replaceable>) will
	be created automatically and should 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 clause.
       </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>1998-09-24</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.  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
   conceptually into groups.
   (See <xref linkend="sql-groupby" endterm="sql-groupby-title">.)
  </para>

  <para>
   The HAVING clause specifies a grouped table derived by the
   elimination of groups from the result of the previously
   specified clause.
   (See <xref linkend="sql-having" endterm="sql-having-title">.)
  </para>
   
  <para>
   The ORDER BY clause allows a user to specify that he/she
   wishes the rows sorted according to the ASCending or 
   DESCending mode operator.
   (See <xref linkend="sql-orderby-title" endterm="sql-orderby-title">.)
  </para>
   
  <para>
   The UNION clause 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 give you the rows that are common to both queries.
   (See <xref linkend="sql-intersect" endterm="sql-intersect-title">.)
  </para>
   
  <para>
   The EXCEPT give you the rows in the upper query not in the lower 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 control over which rows are
   returned by the query.
  </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>1998-09-24</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.
    The comparison returns either TRUE or FALSE and all
    instances will be discarded
    if the expression evaluates to FALSE.
   </para>
  </refsect2>
  
  <refsect2 id="SQL-GROUPBY">
   <refsect2info>
    <date>1998-09-24</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 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 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>
  </refsect2>

  <refsect2 id="SQL-HAVING">
   <refsect2info>
    <date>1998-09-24</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 groups from the result of the previously specified clause
    that do not meet the <replaceable class="PARAMETER">cond_expr</replaceable>.</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>1998-09-24</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 column
    name or an ordinal number.
   </para>
   <para>
    The ordinal numbers refers to the ordinal (left-to-right) position
    of the 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
    assign a name
    to a calculated column using the AS clause, e.g.:
    <programlisting>
SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
    </programlisting></para>
    
   <para>
    From release 6.4 of PostgreSQL, the columns in the ORDER BY clause
    do not need to appear in the SELECT clause.
    Thus the following statement is now legal:
    <programlisting>
SELECT name FROM distributors ORDER BY code;
    </programlisting>
   </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.
   </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 clause.
   </para>
    
   <para>
    The UNION clause allows the result to be the collection of rows
    returned by the queries involved. (See UNION clause).
    The two tables that represent the direct operands of the UNION must
    have 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 clause.
   </para>

   <para>
    The INTERSECT clause allows the result to be all rows that are 
    common to the involved queries.
    The two tables that represent the direct operands of the INTERSECT must
    have 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.
   </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 clause.
   </para>

   <para>
    The EXCEPT clause allows the result to be rows from the upper query
    that are not in the lower query.  (See EXCEPT clause).
    The two tables that represent the direct operands of the EXCEPT must
    have 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.
   </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
   </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
   </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
   </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
   </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 to be used, 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>
     In the <acronym>SQL92</acronym> standard, the new column name 
     specified in an
     "AS" clause may be referenced in GROUP BY and HAVING clauses.
     This is not currently
     allowed in <productname>Postgres</productname>.
    </para>
     
    <para>
     The DISTINCT ON phrase is not part of <acronym>SQL92</acronym>.
    </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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