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df0ea5a1cd
frame clause, as appears to be required by the fine print in the SQL spec. Per discussion with Pavel, not doing so risks user confusion.
1524 lines
61 KiB
Plaintext
1524 lines
61 KiB
Plaintext
<!--
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$PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.116 2008/12/31 23:42:56 tgl Exp $
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PostgreSQL documentation
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-->
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<refentry id="SQL-SELECT">
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<refmeta>
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<refentrytitle id="sql-select-title">SELECT</refentrytitle>
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<manvolnum>7</manvolnum>
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<refmiscinfo>SQL - Language Statements</refmiscinfo>
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</refmeta>
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<refnamediv>
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<refname>SELECT</refname>
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<refname>TABLE</refname>
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<refname>WITH</refname>
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<refpurpose>retrieve rows from a table or view</refpurpose>
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</refnamediv>
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<indexterm zone="sql-select">
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<primary>SELECT</primary>
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</indexterm>
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<indexterm zone="sql-select">
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<primary>TABLE command</primary>
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</indexterm>
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<indexterm zone="sql-select">
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<primary>WITH</primary>
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<secondary>in SELECT</secondary>
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</indexterm>
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<refsynopsisdiv>
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<synopsis>
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[ WITH [ RECURSIVE ] <replaceable class="parameter">with_query</replaceable> [, ...] ]
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SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replaceable> [, ...] ) ] ]
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* | <replaceable class="parameter">expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] [, ...]
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[ FROM <replaceable class="parameter">from_item</replaceable> [, ...] ]
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[ WHERE <replaceable class="parameter">condition</replaceable> ]
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[ GROUP BY <replaceable class="parameter">expression</replaceable> [, ...] ]
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[ HAVING <replaceable class="parameter">condition</replaceable> [, ...] ]
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[ WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceable class="parameter">window_definition</replaceable> ) [, ...] ]
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[ { UNION | INTERSECT | EXCEPT } [ ALL ] <replaceable class="parameter">select</replaceable> ]
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[ ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [ NULLS { FIRST | LAST } ] [, ...] ]
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[ LIMIT { <replaceable class="parameter">count</replaceable> | ALL } ]
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[ OFFSET <replaceable class="parameter">start</replaceable> [ ROW | ROWS ] ]
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[ FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } ONLY ]
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[ FOR { UPDATE | SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT ] [...] ]
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where <replaceable class="parameter">from_item</replaceable> can be one of:
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[ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
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( <replaceable class="parameter">select</replaceable> ) [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ]
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<replaceable class="parameter">with_query_name</replaceable> [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]
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<replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] ) [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] | <replaceable class="parameter">column_definition</replaceable> [, ...] ) ]
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<replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] ) AS ( <replaceable class="parameter">column_definition</replaceable> [, ...] )
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<replaceable class="parameter">from_item</replaceable> [ NATURAL ] <replaceable class="parameter">join_type</replaceable> <replaceable class="parameter">from_item</replaceable> [ ON <replaceable class="parameter">join_condition</replaceable> | USING ( <replaceable class="parameter">join_column</replaceable> [, ...] ) ]
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and <replaceable class="parameter">with_query</replaceable> is:
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<replaceable class="parameter">with_query_name</replaceable> [ ( <replaceable class="parameter">column_name</replaceable> [, ...] ) ] AS ( <replaceable class="parameter">select</replaceable> )
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TABLE { [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] | <replaceable class="parameter">with_query_name</replaceable> }
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</synopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>
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<command>SELECT</command> retrieves rows from zero or more tables.
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The general processing of <command>SELECT</command> is as follows:
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<orderedlist>
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<listitem>
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<para>
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All queries in the <literal>WITH</literal> list are computed.
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These effectively serve as temporary tables that can be referenced
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in the <literal>FROM</literal> list. A <literal>WITH</literal> query
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that is referenced more than once in <literal>FROM</literal> is
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computed only once.
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(See <xref linkend="sql-with" endterm="sql-with-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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All elements in the <literal>FROM</literal> list are computed.
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(Each element in the <literal>FROM</literal> list is a real or
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virtual table.) If more than one element is specified in the
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<literal>FROM</literal> list, they are cross-joined together.
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(See <xref linkend="sql-from" endterm="sql-from-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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If the <literal>WHERE</literal> clause is specified, all rows
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that do not satisfy the condition are eliminated from the
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output. (See <xref linkend="sql-where"
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endterm="sql-where-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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If the <literal>GROUP BY</literal> clause is specified, the
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output is divided into groups of rows that match on one or more
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values. If the <literal>HAVING</literal> clause is present, it
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eliminates groups that do not satisfy the given condition. (See
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<xref linkend="sql-groupby" endterm="sql-groupby-title"> and
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<xref linkend="sql-having" endterm="sql-having-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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The actual output rows are computed using the
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<command>SELECT</command> output expressions for each selected
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row. (See
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<xref linkend="sql-select-list" endterm="sql-select-list-title">
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below.)
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</para>
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</listitem>
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<listitem>
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<para>
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Using the operators <literal>UNION</literal>,
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<literal>INTERSECT</literal>, and <literal>EXCEPT</literal>, the
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output of more than one <command>SELECT</command> statement can
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be combined to form a single result set. The
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<literal>UNION</literal> operator returns all rows that are in
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one or both of the result sets. The
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<literal>INTERSECT</literal> operator returns all rows that are
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strictly in both result sets. The <literal>EXCEPT</literal>
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operator returns the rows that are in the first result set but
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not in the second. In all three cases, duplicate rows are
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eliminated unless <literal>ALL</literal> is specified. (See
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<xref linkend="sql-union" endterm="sql-union-title">, <xref
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linkend="sql-intersect" endterm="sql-intersect-title">, and
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<xref linkend="sql-except" endterm="sql-except-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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If the <literal>ORDER BY</literal> clause is specified, the
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returned rows are sorted in the specified order. If
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<literal>ORDER BY</literal> is not given, the rows are returned
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in whatever order the system finds fastest to produce. (See
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<xref linkend="sql-orderby" endterm="sql-orderby-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>DISTINCT</literal> eliminates duplicate rows from the
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result. <literal>DISTINCT ON</literal> eliminates rows that
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match on all the specified expressions. <literal>ALL</literal>
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(the default) will return all candidate rows, including
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duplicates. (See <xref linkend="sql-distinct"
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endterm="sql-distinct-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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If the <literal>LIMIT</literal> (or <literal>FETCH FIRST</literal>) or <literal>OFFSET</literal>
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clause is specified, the <command>SELECT</command> statement
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only returns a subset of the result rows. (See <xref
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linkend="sql-limit" endterm="sql-limit-title"> below.)
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</para>
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</listitem>
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<listitem>
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<para>
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If <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal>
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is specified, the
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<command>SELECT</command> statement locks the selected rows
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against concurrent updates. (See <xref linkend="sql-for-update-share"
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endterm="sql-for-update-share-title"> below.)
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</para>
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</listitem>
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</orderedlist>
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</para>
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<para>
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You must have <literal>SELECT</literal> privilege on a table to
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read its values. The use of <literal>FOR UPDATE</literal> or
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<literal>FOR SHARE</literal> requires
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<literal>UPDATE</literal> privilege as well.
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</para>
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</refsect1>
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<refsect1>
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<title>Parameters</title>
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<refsect2 id="SQL-WITH">
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<title id="sql-with-title"><literal>WITH</literal> Clause</title>
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<para>
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The <literal>WITH</literal> clause allows you to specify one or more
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subqueries that can be referenced by name in the primary query.
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The subqueries effectively act as temporary tables or views
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for the duration of the primary query.
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</para>
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<para>
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A name (without schema qualification) must be specified for each
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<literal>WITH</literal> query. Optionally, a list of column names
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can be specified; if this is omitted,
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the column names are inferred from the subquery.
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</para>
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<para>
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If <literal>RECURSIVE</literal> is specified, it allows a
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subquery to reference itself by name. Such a subquery must have
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the form
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<synopsis>
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<replaceable class="parameter">non_recursive_term</replaceable> UNION [ ALL ] <replaceable class="parameter">recursive_term</replaceable>
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</synopsis>
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where the recursive self-reference must appear on the right-hand
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side of the <literal>UNION</>. Only one recursive self-reference
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is permitted per query.
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</para>
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<para>
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Another effect of <literal>RECURSIVE</literal> is that
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<literal>WITH</literal> queries need not be ordered: a query
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can reference another one that is later in the list. (However,
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circular references, or mutual recursion, are not implemented.)
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Without <literal>RECURSIVE</literal>, <literal>WITH</literal> queries
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can only reference sibling <literal>WITH</literal> queries
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that are earlier in the <literal>WITH</literal> list.
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</para>
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<para>
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A useful property of <literal>WITH</literal> queries is that they
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are evaluated only once per execution of the primary query,
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even if the primary query refers to them more than once.
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</para>
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<para>
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See <xref linkend="queries-with"> for additional information.
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</para>
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</refsect2>
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<refsect2 id="SQL-FROM">
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<title id="sql-from-title"><literal>FROM</literal> Clause</title>
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<para>
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The <literal>FROM</literal> clause specifies one or more source
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tables for the <command>SELECT</command>. If multiple sources are
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specified, the result is the Cartesian product (cross join) of all
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the sources. But usually qualification conditions
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are added to restrict the returned rows to a small subset of the
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Cartesian product.
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</para>
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<para>
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The <literal>FROM</literal> clause can contain the following
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elements:
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<variablelist>
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<varlistentry>
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<term><replaceable class="parameter">table_name</replaceable></term>
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<listitem>
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<para>
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The name (optionally schema-qualified) of an existing table or
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view. If <literal>ONLY</> is specified, only that table is
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scanned. If <literal>ONLY</> is not specified, the table and
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all its descendant tables (if any) are scanned. <literal>*</>
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can be appended to the table name to indicate that descendant
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tables are to be scanned, but in the current version, this is
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the default behavior. (In releases before 7.1,
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<literal>ONLY</> was the default behavior.) The default
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behavior can be modified by changing the <xref
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linkend="guc-sql-inheritance"> configuration option.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">alias</replaceable></term>
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<listitem>
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<para>
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A substitute name for the <literal>FROM</> item containing the
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alias. An alias is used for brevity or to eliminate ambiguity
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for self-joins (where the same table is scanned multiple
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times). When an alias is provided, it completely hides the
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actual name of the table or function; for example given
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<literal>FROM foo AS f</>, the remainder of the
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<command>SELECT</command> must refer to this <literal>FROM</>
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item as <literal>f</> not <literal>foo</>. If an alias is
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written, a column alias list can also be written to provide
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substitute names for one or more columns of the table.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">select</replaceable></term>
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<listitem>
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<para>
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A sub-<command>SELECT</command> can appear in the
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<literal>FROM</literal> clause. This acts as though its
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output were created as a temporary table for the duration of
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this single <command>SELECT</command> command. Note that the
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sub-<command>SELECT</command> must be surrounded by
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parentheses, and an alias <emphasis>must</emphasis> be
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provided for it. A
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<xref linkend="sql-values" endterm="sql-values-title"> command
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can also be used here.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">with_query_name</replaceable></term>
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<listitem>
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<para>
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A <literal>WITH</> query is referenced by writing its name,
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just as though the query's name were a table name. (In fact,
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the <literal>WITH</> query hides any real table of the same name
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for the purposes of the primary query. If necessary, you can
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refer to a real table of the same name by schema-qualifying
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the table's name.)
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An alias can be provided in the same way as for a table.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">function_name</replaceable></term>
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<listitem>
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<para>
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Function calls can appear in the <literal>FROM</literal>
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clause. (This is especially useful for functions that return
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result sets, but any function can be used.) This acts as
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though its output were created as a temporary table for the
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duration of this single <command>SELECT</command> command. An
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alias can also be used. If an alias is written, a column alias
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list can also be written to provide substitute names for one
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or more attributes of the function's composite return type. If
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the function has been defined as returning the <type>record</>
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data type, then an alias or the key word <literal>AS</> must
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be present, followed by a column definition list in the form
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<literal>( <replaceable
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class="parameter">column_name</replaceable> <replaceable
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class="parameter">data_type</replaceable> <optional>, ... </>
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)</literal>. The column definition list must match the actual
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number and types of columns returned by the function.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><replaceable class="parameter">join_type</replaceable></term>
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<listitem>
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<para>
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One of
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<itemizedlist>
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<listitem>
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<para><literal>[ INNER ] JOIN</literal></para>
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</listitem>
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<listitem>
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<para><literal>LEFT [ OUTER ] JOIN</literal></para>
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</listitem>
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<listitem>
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<para><literal>RIGHT [ OUTER ] JOIN</literal></para>
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</listitem>
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<listitem>
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<para><literal>FULL [ OUTER ] JOIN</literal></para>
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</listitem>
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<listitem>
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<para><literal>CROSS JOIN</literal></para>
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</listitem>
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</itemizedlist>
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For the <literal>INNER</> and <literal>OUTER</> join types, a
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join condition must be specified, namely exactly one of
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<literal>NATURAL</>, <literal>ON <replaceable
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|
class="parameter">join_condition</replaceable></literal>, or
|
|
<literal>USING (<replaceable
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|
class="parameter">join_column</replaceable> [, ...])</literal>.
|
|
See below for the meaning. For <literal>CROSS JOIN</literal>,
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none of these clauses can appear.
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</para>
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<para>
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A <literal>JOIN</literal> clause combines two
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<literal>FROM</> items. Use parentheses if necessary to
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determine the order of nesting. In the absence of parentheses,
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<literal>JOIN</literal>s nest left-to-right. In any case
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<literal>JOIN</literal> binds more tightly than the commas
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|
separating <literal>FROM</> items.
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|
</para>
|
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|
|
<para>
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|
<literal>CROSS JOIN</> and <literal>INNER JOIN</literal>
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|
produce a simple Cartesian product, the same result as you get from
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listing the two items at the top level of <literal>FROM</>,
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but restricted by the join condition (if any).
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<literal>CROSS JOIN</> is equivalent to <literal>INNER JOIN ON
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(TRUE)</>, that is, no rows are removed by qualification.
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|
These join types are just a notational convenience, since they
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|
do nothing you couldn't do with plain <literal>FROM</> and
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|
<literal>WHERE</>.
|
|
</para>
|
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|
|
<para>
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|
<literal>LEFT OUTER JOIN</> returns all rows in the qualified
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Cartesian product (i.e., all combined rows that pass its join
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condition), plus one copy of each row in the left-hand table
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|
for which there was no right-hand row that passed the join
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|
condition. This left-hand row is extended to the full width
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|
of the joined table by inserting null values for the
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right-hand columns. Note that only the <literal>JOIN</>
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|
clause's own condition is considered while deciding which rows
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have matches. Outer conditions are applied afterwards.
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</para>
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<para>
|
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Conversely, <literal>RIGHT OUTER JOIN</> returns all the
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joined rows, plus one row for each unmatched right-hand row
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(extended with nulls on the left). This is just a notational
|
|
convenience, since you could convert it to a <literal>LEFT
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OUTER JOIN</> by switching the left and right inputs.
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</para>
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<para>
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<literal>FULL OUTER JOIN</> returns all the joined rows, plus
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one row for each unmatched left-hand row (extended with nulls
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on the right), plus one row for each unmatched right-hand row
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(extended with nulls on the left).
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</para>
|
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</listitem>
|
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</varlistentry>
|
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|
|
<varlistentry>
|
|
<term><literal>ON <replaceable class="parameter">join_condition</replaceable></literal></term>
|
|
<listitem>
|
|
<para>
|
|
<replaceable class="parameter">join_condition</replaceable> is
|
|
an expression resulting in a value of type
|
|
<type>boolean</type> (similar to a <literal>WHERE</literal>
|
|
clause) that specifies which rows in a join are considered to
|
|
match.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>USING ( <replaceable class="parameter">join_column</replaceable> [, ...] )</literal></term>
|
|
<listitem>
|
|
<para>
|
|
A clause of the form <literal>USING ( a, b, ... )</literal> is
|
|
shorthand for <literal>ON left_table.a = right_table.a AND
|
|
left_table.b = right_table.b ...</literal>. Also,
|
|
<literal>USING</> implies that only one of each pair of
|
|
equivalent columns will be included in the join output, not
|
|
both.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><literal>NATURAL</literal></term>
|
|
<listitem>
|
|
<para>
|
|
<literal>NATURAL</literal> is shorthand for a
|
|
<literal>USING</> list that mentions all columns in the two
|
|
tables that have the same names.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-WHERE">
|
|
<title id="sql-where-title"><literal>WHERE</literal> Clause</title>
|
|
|
|
<para>
|
|
The optional <literal>WHERE</literal> clause has the general form
|
|
<synopsis>
|
|
WHERE <replaceable class="parameter">condition</replaceable>
|
|
</synopsis>
|
|
where <replaceable class="parameter">condition</replaceable> is
|
|
any expression that evaluates to a result of type
|
|
<type>boolean</type>. Any row that does not satisfy this
|
|
condition will be eliminated from the output. A row satisfies the
|
|
condition if it returns true when the actual row values are
|
|
substituted for any variable references.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-GROUPBY">
|
|
<title id="sql-groupby-title"><literal>GROUP BY</literal> Clause</title>
|
|
|
|
<para>
|
|
The optional <literal>GROUP BY</literal> clause has the general form
|
|
<synopsis>
|
|
GROUP BY <replaceable class="parameter">expression</replaceable> [, ...]
|
|
</synopsis>
|
|
</para>
|
|
|
|
<para>
|
|
<literal>GROUP BY</literal> will condense into a single row all
|
|
selected rows that share the same values for the grouped
|
|
expressions. <replaceable
|
|
class="parameter">expression</replaceable> can be an input column
|
|
name, or the name or ordinal number of an output column
|
|
(<command>SELECT</command> list item), or an arbitrary
|
|
expression formed from input-column values. In case of ambiguity,
|
|
a <literal>GROUP BY</literal> name will be interpreted as an
|
|
input-column name rather than an output column name.
|
|
</para>
|
|
|
|
<para>
|
|
Aggregate functions, if any are used, are computed across all rows
|
|
making up each group, producing a separate value for each group
|
|
(whereas without <literal>GROUP BY</literal>, an aggregate
|
|
produces a single value computed across all the selected rows).
|
|
When <literal>GROUP BY</literal> is present, it is not valid for
|
|
the <command>SELECT</command> list expressions 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">
|
|
<title id="sql-having-title"><literal>HAVING</literal> Clause</title>
|
|
|
|
<para>
|
|
The optional <literal>HAVING</literal> clause has the general form
|
|
<synopsis>
|
|
HAVING <replaceable class="parameter">condition</replaceable>
|
|
</synopsis>
|
|
where <replaceable class="parameter">condition</replaceable> is
|
|
the same as specified for the <literal>WHERE</literal> clause.
|
|
</para>
|
|
|
|
<para>
|
|
<literal>HAVING</literal> eliminates group rows that do not
|
|
satisfy the condition. <literal>HAVING</literal> is different
|
|
from <literal>WHERE</literal>: <literal>WHERE</literal> filters
|
|
individual rows before the application of <literal>GROUP
|
|
BY</literal>, while <literal>HAVING</literal> filters group rows
|
|
created by <literal>GROUP BY</literal>. Each column referenced in
|
|
<replaceable class="parameter">condition</replaceable> must
|
|
unambiguously reference a grouping column, unless the reference
|
|
appears within an aggregate function.
|
|
</para>
|
|
|
|
<para>
|
|
The presence of <literal>HAVING</literal> turns a query into a grouped
|
|
query even if there is no <literal>GROUP BY</> clause. This is the
|
|
same as what happens when the query contains aggregate functions but
|
|
no <literal>GROUP BY</> clause. All the selected rows are considered to
|
|
form a single group, and the <command>SELECT</command> list and
|
|
<literal>HAVING</literal> clause can only reference table columns from
|
|
within aggregate functions. Such a query will emit a single row if the
|
|
<literal>HAVING</literal> condition is true, zero rows if it is not true.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-WINDOW">
|
|
<title id="sql-window-title"><literal>WINDOW</literal> Clause</title>
|
|
|
|
<para>
|
|
The optional <literal>WINDOW</literal> clause has the general form
|
|
<synopsis>
|
|
WINDOW <replaceable class="parameter">window_name</replaceable> AS ( <replaceable class="parameter">window_definition</replaceable> ) [, ...]
|
|
</synopsis>
|
|
where <replaceable class="parameter">window_name</replaceable> is
|
|
a name that can be referenced from subsequent window definitions or
|
|
<literal>OVER</> clauses, and
|
|
<replaceable class="parameter">window_definition</replaceable> is
|
|
<synopsis>
|
|
[ <replaceable class="parameter">existing_window_name</replaceable> ]
|
|
[ PARTITION BY <replaceable class="parameter">expression</replaceable> [, ...] ]
|
|
[ ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [ NULLS { FIRST | LAST } ] [, ...] ]
|
|
[ <replaceable class="parameter">frame_clause</replaceable> ]
|
|
</synopsis>
|
|
</para>
|
|
|
|
<para>
|
|
If an <replaceable class="parameter">existing_window_name</replaceable>
|
|
is specified it must refer to an earlier entry in the <literal>WINDOW</>
|
|
list; the new window copies its partitioning clause from that entry,
|
|
as well as its ordering clause if any. In this case the new window cannot
|
|
specify its own <literal>PARTITION BY</> clause, and it can specify
|
|
<literal>ORDER BY</> only if the copied window does not have one.
|
|
The new window always uses its own frame clause; the copied window
|
|
must not specify a frame clause.
|
|
</para>
|
|
|
|
<para>
|
|
The elements of the <literal>PARTITION BY</> list are interpreted in
|
|
the same fashion as elements of a
|
|
<xref linkend="sql-groupby" endterm="sql-groupby-title">, and
|
|
the elements of the <literal>ORDER BY</> list are interpreted in the
|
|
same fashion as elements of an
|
|
<xref linkend="sql-orderby" endterm="sql-orderby-title">.
|
|
The only difference is that these expressions can contain aggregate
|
|
function calls, which are not allowed in a regular <literal>GROUP BY</>
|
|
clause. They are allowed here because windowing occurs after grouping
|
|
and aggregation.
|
|
</para>
|
|
|
|
<para>
|
|
The optional <replaceable class="parameter">frame_clause</> defines
|
|
the <firstterm>window frame</> for window functions that depend on the
|
|
frame (not all do). It can be one of
|
|
<synopsis>
|
|
RANGE UNBOUNDED PRECEDING
|
|
RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
|
|
RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
|
|
ROWS UNBOUNDED PRECEDING
|
|
ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
|
|
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
|
|
</synopsis>
|
|
The first two are equivalent and are also the default: they set the
|
|
frame to be all rows from the partition start up through the current row's
|
|
last peer in the <literal>ORDER BY</> ordering (which means all rows if
|
|
there is no <literal>ORDER BY</>). The options
|
|
<literal>RANGE BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</> and
|
|
<literal>ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING</>
|
|
are also equivalent: they always select all rows in the partition.
|
|
Lastly, <literal>ROWS UNBOUNDED PRECEDING</> or its verbose equivalent
|
|
<literal>ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW</> select
|
|
all rows up through the current row (regardless of duplicates).
|
|
Beware that this option can produce implementation-dependent results
|
|
if the <literal>ORDER BY</> ordering does not order the rows uniquely.
|
|
</para>
|
|
|
|
<para>
|
|
The purpose of a <literal>WINDOW</literal> clause is to specify the
|
|
behavior of <firstterm>window functions</> appearing in the query's
|
|
<xref linkend="sql-select-list" endterm="sql-select-list-title"> or
|
|
<xref linkend="sql-orderby" endterm="sql-orderby-title">. These functions
|
|
can reference the <literal>WINDOW</literal> clause entries by name
|
|
in their <literal>OVER</> clauses. A <literal>WINDOW</literal> clause
|
|
entry does not have to be referenced anywhere, however; if it is not
|
|
used in the query it is simply ignored. It is possible to use window
|
|
functions without any <literal>WINDOW</literal> clause at all, since
|
|
a window function call can specify its window definition directly in
|
|
its <literal>OVER</> clause. However, the <literal>WINDOW</literal>
|
|
clause saves typing when the same window definition is needed for more
|
|
than one window function.
|
|
</para>
|
|
|
|
<para>
|
|
Window functions are described in detail in
|
|
<xref linkend="tutorial-window">,
|
|
<xref linkend="syntax-window-functions">, and
|
|
<xref linkend="queries-window">.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="sql-select-list">
|
|
<title id="sql-select-list-title"><command>SELECT</command> List</title>
|
|
|
|
<para>
|
|
The <command>SELECT</command> list (between the key words
|
|
<literal>SELECT</> and <literal>FROM</>) specifies expressions
|
|
that form the output rows of the <command>SELECT</command>
|
|
statement. The expressions can (and usually do) refer to columns
|
|
computed in the <literal>FROM</> clause.
|
|
</para>
|
|
|
|
<para>
|
|
Just as in a table, every output column of a <command>SELECT</command>
|
|
has a name. In a simple <command>SELECT</command> this name is just
|
|
used to label the column for display, but when the <command>SELECT</>
|
|
is a sub-query of a larger query, the name is seen by the larger query
|
|
as the column name of the virtual table produced by the sub-query.
|
|
To specify the name to use for an output column, write
|
|
<literal>AS</> <replaceable class="parameter">output_name</replaceable>
|
|
after the column's expression. (You can omit <literal>AS</literal>,
|
|
but only if the desired output name does not match any
|
|
<productname>PostgreSQL</productname> keyword (see <xref
|
|
linkend="sql-keywords-appendix">). For protection against possible
|
|
future keyword additions, it is recommended that you always either
|
|
write <literal>AS</literal> or double-quote the output name.)
|
|
If you do not specify a column name, a name is chosen automatically
|
|
by <productname>PostgreSQL</productname>. If the column's expression
|
|
is a simple column reference then the chosen name is the same as that
|
|
column's name; in more complex cases a generated name looking like
|
|
<literal>?column<replaceable>N</>?</literal> is usually chosen.
|
|
</para>
|
|
|
|
<para>
|
|
An output column's name can be used to refer to the column's value in
|
|
<literal>ORDER BY</> and <literal>GROUP BY</> clauses, but not in the
|
|
<literal>WHERE</> or <literal>HAVING</> clauses; there you must write
|
|
out the expression instead.
|
|
</para>
|
|
|
|
<para>
|
|
Instead of an expression, <literal>*</literal> can be written in
|
|
the output list as a shorthand for all the columns of the selected
|
|
rows. Also, you can write <literal><replaceable
|
|
class="parameter">table_name</replaceable>.*</literal> as a
|
|
shorthand for the columns coming from just that table. In these
|
|
cases it is not possible to specify new names with <literal>AS</>;
|
|
the output column names will be the same as the table columns' names.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-UNION">
|
|
<title id="sql-union-title"><literal>UNION</literal> Clause</title>
|
|
|
|
<para>
|
|
The <literal>UNION</literal> clause has this general form:
|
|
<synopsis>
|
|
<replaceable class="parameter">select_statement</replaceable> UNION [ ALL ] <replaceable class="parameter">select_statement</replaceable>
|
|
</synopsis>
|
|
<replaceable class="parameter">select_statement</replaceable> is
|
|
any <command>SELECT</command> statement without an <literal>ORDER
|
|
BY</>, <literal>LIMIT</>, <literal>FOR UPDATE</literal>, or
|
|
<literal>FOR SHARE</literal> clause.
|
|
(<literal>ORDER BY</> and <literal>LIMIT</> can be attached to a
|
|
subexpression if it is enclosed in parentheses. Without
|
|
parentheses, these clauses will be taken to apply to the result of
|
|
the <literal>UNION</literal>, not to its right-hand input
|
|
expression.)
|
|
</para>
|
|
|
|
<para>
|
|
The <literal>UNION</literal> operator computes the set union of
|
|
the rows returned by the involved <command>SELECT</command>
|
|
statements. A row is in the set union of two result sets if it
|
|
appears in at least one of the result sets. The two
|
|
<command>SELECT</command> statements that represent the direct
|
|
operands of the <literal>UNION</literal> must produce the same
|
|
number of columns, and corresponding columns must be of compatible
|
|
data types.
|
|
</para>
|
|
|
|
<para>
|
|
The result of <literal>UNION</> does not contain any duplicate
|
|
rows unless the <literal>ALL</> option is specified.
|
|
<literal>ALL</> prevents elimination of duplicates. (Therefore,
|
|
<literal>UNION ALL</> is usually significantly quicker than
|
|
<literal>UNION</>; use <literal>ALL</> when you can.)
|
|
</para>
|
|
|
|
<para>
|
|
Multiple <literal>UNION</> operators in the same
|
|
<command>SELECT</command> statement are evaluated left to right,
|
|
unless otherwise indicated by parentheses.
|
|
</para>
|
|
|
|
<para>
|
|
Currently, <literal>FOR UPDATE</> and <literal>FOR SHARE</> cannot be
|
|
specified either for a <literal>UNION</> result or for any input of a
|
|
<literal>UNION</>.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-INTERSECT">
|
|
<title id="sql-intersect-title"><literal>INTERSECT</literal> Clause</title>
|
|
|
|
<para>
|
|
The <literal>INTERSECT</literal> clause has this general form:
|
|
<synopsis>
|
|
<replaceable class="parameter">select_statement</replaceable> INTERSECT [ ALL ] <replaceable class="parameter">select_statement</replaceable>
|
|
</synopsis>
|
|
<replaceable class="parameter">select_statement</replaceable> is
|
|
any <command>SELECT</command> statement without an <literal>ORDER
|
|
BY</>, <literal>LIMIT</>, <literal>FOR UPDATE</literal>, or
|
|
<literal>FOR SHARE</literal> clause.
|
|
</para>
|
|
|
|
<para>
|
|
The <literal>INTERSECT</literal> operator computes the set
|
|
intersection of the rows returned by the involved
|
|
<command>SELECT</command> statements. A row is in the
|
|
intersection of two result sets if it appears in both result sets.
|
|
</para>
|
|
|
|
<para>
|
|
The result of <literal>INTERSECT</literal> does not contain any
|
|
duplicate rows unless the <literal>ALL</> option is specified.
|
|
With <literal>ALL</>, a row that has <replaceable>m</> duplicates in the
|
|
left table and <replaceable>n</> duplicates in the right table will appear
|
|
min(<replaceable>m</>,<replaceable>n</>) times in the result set.
|
|
</para>
|
|
|
|
<para>
|
|
Multiple <literal>INTERSECT</literal> operators in the same
|
|
<command>SELECT</command> statement are evaluated left to right,
|
|
unless parentheses dictate otherwise.
|
|
<literal>INTERSECT</literal> binds more tightly than
|
|
<literal>UNION</literal>. That is, <literal>A UNION B INTERSECT
|
|
C</literal> will be read as <literal>A UNION (B INTERSECT
|
|
C)</literal>.
|
|
</para>
|
|
|
|
<para>
|
|
Currently, <literal>FOR UPDATE</> and <literal>FOR SHARE</> cannot be
|
|
specified either for an <literal>INTERSECT</> result or for any input of
|
|
an <literal>INTERSECT</>.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-EXCEPT">
|
|
<title id="sql-except-title"><literal>EXCEPT</literal> Clause</title>
|
|
|
|
<para>
|
|
The <literal>EXCEPT</literal> clause has this general form:
|
|
<synopsis>
|
|
<replaceable class="parameter">select_statement</replaceable> EXCEPT [ ALL ] <replaceable class="parameter">select_statement</replaceable>
|
|
</synopsis>
|
|
<replaceable class="parameter">select_statement</replaceable> is
|
|
any <command>SELECT</command> statement without an <literal>ORDER
|
|
BY</>, <literal>LIMIT</>, <literal>FOR UPDATE</literal>, or
|
|
<literal>FOR SHARE</literal> clause.
|
|
</para>
|
|
|
|
<para>
|
|
The <literal>EXCEPT</literal> operator computes the set of rows
|
|
that are in the result of the left <command>SELECT</command>
|
|
statement but not in the result of the right one.
|
|
</para>
|
|
|
|
<para>
|
|
The result of <literal>EXCEPT</literal> does not contain any
|
|
duplicate rows unless the <literal>ALL</> option is specified.
|
|
With <literal>ALL</>, a row that has <replaceable>m</> duplicates in the
|
|
left table and <replaceable>n</> duplicates in the right table will appear
|
|
max(<replaceable>m</>-<replaceable>n</>,0) times in the result set.
|
|
</para>
|
|
|
|
<para>
|
|
Multiple <literal>EXCEPT</literal> operators in the same
|
|
<command>SELECT</command> statement are evaluated left to right,
|
|
unless parentheses dictate otherwise. <literal>EXCEPT</> binds at
|
|
the same level as <literal>UNION</>.
|
|
</para>
|
|
|
|
<para>
|
|
Currently, <literal>FOR UPDATE</> and <literal>FOR SHARE</> cannot be
|
|
specified either for an <literal>EXCEPT</> result or for any input of
|
|
an <literal>EXCEPT</>.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-ORDERBY">
|
|
<title id="sql-orderby-title"><literal>ORDER BY</literal> Clause</title>
|
|
|
|
<para>
|
|
The optional <literal>ORDER BY</literal> clause has this general form:
|
|
<synopsis>
|
|
ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [ NULLS { FIRST | LAST } ] [, ...]
|
|
</synopsis>
|
|
The <literal>ORDER BY</literal> clause causes the result rows to
|
|
be sorted according to the specified expression(s). If two rows are
|
|
equal according to the leftmost expression, they are compared
|
|
according to the next expression and so on. If they are equal
|
|
according to all specified expressions, they are returned in
|
|
an implementation-dependent order.
|
|
</para>
|
|
|
|
<para>
|
|
Each <replaceable class="parameter">expression</replaceable> can be the
|
|
name or ordinal number of an output column
|
|
(<command>SELECT</command> list item), or it can be an arbitrary
|
|
expression formed from input-column values.
|
|
</para>
|
|
|
|
<para>
|
|
The ordinal number refers to the ordinal (left-to-right) position
|
|
of the output column. This feature makes it possible to define an
|
|
ordering on the basis of a column that does not have a unique
|
|
name. This is never absolutely necessary because it is always
|
|
possible to assign a name to an output column using the
|
|
<literal>AS</> clause.
|
|
</para>
|
|
|
|
<para>
|
|
It is also possible to use arbitrary expressions in the
|
|
<literal>ORDER BY</literal> clause, including columns that do not
|
|
appear in the <command>SELECT</command> output list. Thus the
|
|
following statement is valid:
|
|
<programlisting>
|
|
SELECT name FROM distributors ORDER BY code;
|
|
</programlisting>
|
|
A limitation of this feature is that an <literal>ORDER BY</>
|
|
clause applying to the result of a <literal>UNION</>,
|
|
<literal>INTERSECT</>, or <literal>EXCEPT</> clause can only
|
|
specify an output column name or number, not an expression.
|
|
</para>
|
|
|
|
<para>
|
|
If an <literal>ORDER BY</> expression is a simple name that
|
|
matches both an output column name and an input column name,
|
|
<literal>ORDER BY</> will interpret it as the output column name.
|
|
This is the opposite of the choice that <literal>GROUP BY</> will
|
|
make in the same situation. This inconsistency is made to be
|
|
compatible with the SQL standard.
|
|
</para>
|
|
|
|
<para>
|
|
Optionally one can add the key word <literal>ASC</> (ascending) or
|
|
<literal>DESC</> (descending) after any expression in the
|
|
<literal>ORDER BY</> clause. If not specified, <literal>ASC</> is
|
|
assumed by default. Alternatively, a specific ordering operator
|
|
name can be specified in the <literal>USING</> clause.
|
|
An ordering operator must be a less-than or greater-than
|
|
member of some B-tree operator family.
|
|
<literal>ASC</> is usually equivalent to <literal>USING <</> and
|
|
<literal>DESC</> is usually equivalent to <literal>USING ></>.
|
|
(But the creator of a user-defined data type can define exactly what the
|
|
default sort ordering is, and it might correspond to operators with other
|
|
names.)
|
|
</para>
|
|
|
|
<para>
|
|
If <literal>NULLS LAST</> is specified, null values sort after all
|
|
non-null values; if <literal>NULLS FIRST</> is specified, null values
|
|
sort before all non-null values. If neither is specified, the default
|
|
behavior is <literal>NULLS LAST</> when <literal>ASC</> is specified
|
|
or implied, and <literal>NULLS FIRST</> when <literal>DESC</> is specified
|
|
(thus, the default is to act as though nulls are larger than non-nulls).
|
|
When <literal>USING</> is specified, the default nulls ordering depends
|
|
on whether the operator is a less-than or greater-than operator.
|
|
</para>
|
|
|
|
<para>
|
|
Note that ordering options apply only to the expression they follow;
|
|
for example <literal>ORDER BY x, y DESC</> does not mean
|
|
the same thing as <literal>ORDER BY x DESC, y DESC</>.
|
|
</para>
|
|
|
|
<para>
|
|
Character-string data is sorted according to the locale-specific
|
|
collation order that was established when the database was created.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="sql-distinct">
|
|
<title id="sql-distinct-title"><literal>DISTINCT</literal> Clause</title>
|
|
|
|
<para>
|
|
If <literal>DISTINCT</> is specified, all duplicate rows are
|
|
removed from the result set (one row is kept from each group of
|
|
duplicates). <literal>ALL</> specifies the opposite: all rows are
|
|
kept; that is the default.
|
|
</para>
|
|
|
|
<para>
|
|
<literal>DISTINCT ON ( <replaceable
|
|
class="parameter">expression</replaceable> [, ...] )</literal>
|
|
keeps only the first row of each set of rows where the given
|
|
expressions evaluate to equal. The <literal>DISTINCT ON</literal>
|
|
expressions are interpreted using the same rules as for
|
|
<literal>ORDER BY</> (see above). Note that the <quote>first
|
|
row</quote> of each set is unpredictable unless <literal>ORDER
|
|
BY</> is used to ensure that the desired row appears first. For
|
|
example:
|
|
<programlisting>
|
|
SELECT DISTINCT ON (location) location, time, report
|
|
FROM weather_reports
|
|
ORDER BY location, time DESC;
|
|
</programlisting>
|
|
retrieves the most recent weather report for each location. But
|
|
if we had not used <literal>ORDER BY</> to force descending order
|
|
of time values for each location, we'd have gotten a report from
|
|
an unpredictable time for each location.
|
|
</para>
|
|
|
|
<para>
|
|
The <literal>DISTINCT ON</> expression(s) must match the leftmost
|
|
<literal>ORDER BY</> expression(s). The <literal>ORDER BY</> clause
|
|
will normally contain additional expression(s) that determine the
|
|
desired precedence of rows within each <literal>DISTINCT ON</> group.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-LIMIT">
|
|
<title id="sql-limit-title"><literal>LIMIT</literal> Clause</title>
|
|
|
|
<para>
|
|
The <literal>LIMIT</literal> clause consists of two independent
|
|
sub-clauses:
|
|
<synopsis>
|
|
LIMIT { <replaceable class="parameter">count</replaceable> | ALL }
|
|
OFFSET <replaceable class="parameter">start</replaceable>
|
|
</synopsis>
|
|
<replaceable class="parameter">count</replaceable> specifies the
|
|
maximum number of rows to return, while <replaceable
|
|
class="parameter">start</replaceable> specifies the number of rows
|
|
to skip before starting to return rows. When both are specified,
|
|
<replaceable class="parameter">start</replaceable> rows are skipped
|
|
before starting to count the <replaceable
|
|
class="parameter">count</replaceable> rows to be returned.
|
|
</para>
|
|
|
|
<para>
|
|
SQL:2008 introduced a different syntax to achieve the same thing,
|
|
which PostgreSQL also supports. It is:
|
|
<synopsis>
|
|
OFFSET <replaceable class="parameter">start</replaceable> { ROW | ROWS }
|
|
FETCH { FIRST | NEXT } [ <replaceable class="parameter">count</replaceable> ] { ROW | ROWS } ONLY
|
|
</synopsis>
|
|
Both clauses are optional, but if present
|
|
the <literal>OFFSET</literal> clause must come before
|
|
the <literal>FETCH</literal> clause. <literal>ROW</literal>
|
|
and <literal>ROWS</literal> as well as <literal>FIRST</literal>
|
|
and <literal>NEXT</literal> are noise words that don't influence
|
|
the effects of these clauses. When using expressions other than
|
|
constants for the offset or fetch count, parentheses will be
|
|
necessary in most cases. If the fetch count is omitted, it
|
|
defaults to 1.
|
|
</para>
|
|
|
|
<para>
|
|
When using <literal>LIMIT</>, it is a good idea to use an
|
|
<literal>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 might be asking for the tenth through
|
|
twentieth rows, but tenth through twentieth in what ordering? You
|
|
don't know what ordering unless you specify <literal>ORDER BY</>.
|
|
</para>
|
|
|
|
<para>
|
|
The query planner takes <literal>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 use
|
|
for <literal>LIMIT</> and <literal>OFFSET</>. Thus, using
|
|
different <literal>LIMIT</>/<literal>OFFSET</> values to select
|
|
different subsets of a query result <emphasis>will give
|
|
inconsistent results</emphasis> unless you enforce a predictable
|
|
result ordering with <literal>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
|
|
<literal>ORDER BY</> is used to constrain the order.
|
|
</para>
|
|
|
|
<para>
|
|
It is even possible for repeated executions of the same <literal>LIMIT</>
|
|
query to return different subsets of the rows of a table, if there
|
|
is not an <literal>ORDER BY</> to enforce selection of a deterministic
|
|
subset. Again, this is not a bug; determinism of the results is
|
|
simply not guaranteed in such a case.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-FOR-UPDATE-SHARE">
|
|
<title id="sql-for-update-share-title"><literal>FOR UPDATE</literal>/<literal>FOR SHARE</literal> Clause</title>
|
|
|
|
<para>
|
|
The <literal>FOR UPDATE</literal> clause has this form:
|
|
<synopsis>
|
|
FOR UPDATE [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT ]
|
|
</synopsis>
|
|
</para>
|
|
|
|
<para>
|
|
The closely related <literal>FOR SHARE</literal> clause has this form:
|
|
<synopsis>
|
|
FOR SHARE [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT ]
|
|
</synopsis>
|
|
</para>
|
|
|
|
<para>
|
|
<literal>FOR UPDATE</literal> causes the rows retrieved by the
|
|
<command>SELECT</command> statement to be locked as though for
|
|
update. This prevents them from being modified or deleted by
|
|
other transactions until the current transaction ends. That is,
|
|
other transactions that attempt <command>UPDATE</command>,
|
|
<command>DELETE</command>, or <command>SELECT FOR UPDATE</command>
|
|
of these rows will be blocked until the current transaction ends.
|
|
Also, if an <command>UPDATE</command>, <command>DELETE</command>,
|
|
or <command>SELECT FOR UPDATE</command> from another transaction
|
|
has already locked a selected row or rows, <command>SELECT FOR
|
|
UPDATE</command> will wait for the other transaction to complete,
|
|
and will then lock and return the updated row (or no row, if the
|
|
row was deleted). For further discussion see <xref
|
|
linkend="mvcc">.
|
|
</para>
|
|
|
|
<para>
|
|
To prevent the operation from waiting for other transactions to commit,
|
|
use the <literal>NOWAIT</> option. <command>SELECT FOR UPDATE
|
|
NOWAIT</command> reports an error, rather than waiting, if a selected row
|
|
cannot be locked immediately. Note that <literal>NOWAIT</> applies only
|
|
to the row-level lock(s) — the required <literal>ROW SHARE</literal>
|
|
table-level lock is still taken in the ordinary way (see
|
|
<xref linkend="mvcc">). You can use the <literal>NOWAIT</> option of
|
|
<xref linkend="sql-lock" endterm="sql-lock-title">
|
|
if you need to acquire the table-level lock without waiting.
|
|
</para>
|
|
|
|
<para>
|
|
<literal>FOR SHARE</literal> behaves similarly, except that it
|
|
acquires a shared rather than exclusive lock on each retrieved
|
|
row. A shared lock blocks other transactions from performing
|
|
<command>UPDATE</command>, <command>DELETE</command>, or <command>SELECT
|
|
FOR UPDATE</command> on these rows, but it does not prevent them
|
|
from performing <command>SELECT FOR SHARE</command>.
|
|
</para>
|
|
|
|
<para>
|
|
If specific tables are named in <literal>FOR UPDATE</literal>
|
|
or <literal>FOR SHARE</literal>,
|
|
then only rows coming from those tables are locked; any other
|
|
tables used in the <command>SELECT</command> are simply read as
|
|
usual. A <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal>
|
|
clause without a table list affects all tables used in the command.
|
|
If <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal> is
|
|
applied to a view or sub-query, it affects all tables used in
|
|
the view or sub-query.
|
|
</para>
|
|
|
|
<para>
|
|
Multiple <literal>FOR UPDATE</literal> and <literal>FOR SHARE</literal>
|
|
clauses can be written if it is necessary to specify different locking
|
|
behavior for different tables. If the same table is mentioned (or
|
|
implicitly affected) by both <literal>FOR UPDATE</literal> and
|
|
<literal>FOR SHARE</literal> clauses, then it is processed as
|
|
<literal>FOR UPDATE</literal>. Similarly, a table is processed
|
|
as <literal>NOWAIT</> if that is specified in any of the clauses
|
|
affecting it.
|
|
</para>
|
|
|
|
<para>
|
|
<literal>FOR UPDATE</literal> and <literal>FOR SHARE</literal> cannot be
|
|
used in contexts where returned rows cannot be clearly identified with
|
|
individual table rows; for example they cannot be used with aggregation.
|
|
</para>
|
|
|
|
<caution>
|
|
<para>
|
|
Avoid locking a row and then modifying it within a later savepoint or
|
|
<application>PL/pgSQL</application> exception block. A subsequent
|
|
rollback would cause the lock to be lost. For example:
|
|
<programlisting>
|
|
BEGIN;
|
|
SELECT * FROM mytable WHERE key = 1 FOR UPDATE;
|
|
SAVEPOINT s;
|
|
UPDATE mytable SET ... WHERE key = 1;
|
|
ROLLBACK TO s;
|
|
</programlisting>
|
|
After the <command>ROLLBACK</>, the row is effectively unlocked, rather
|
|
than returned to its pre-savepoint state of being locked but not modified.
|
|
This hazard occurs if a row locked in the current transaction is updated
|
|
or deleted, or if a shared lock is upgraded to exclusive: in all these
|
|
cases, the former lock state is forgotten. If the transaction is then
|
|
rolled back to a state between the original locking command and the
|
|
subsequent change, the row will appear not to be locked at all. This is
|
|
an implementation deficiency which will be addressed in a future release
|
|
of <productname>PostgreSQL</productname>.
|
|
</para>
|
|
</caution>
|
|
|
|
<caution>
|
|
<para>
|
|
It is possible for a <command>SELECT</> command using both
|
|
<literal>LIMIT</literal> and <literal>FOR UPDATE/SHARE</literal>
|
|
clauses to return fewer rows than specified by <literal>LIMIT</literal>.
|
|
This is because <literal>LIMIT</> is applied first. The command
|
|
selects the specified number of rows,
|
|
but might then block trying to obtain lock on one or more of them.
|
|
Once the <literal>SELECT</> unblocks, the row might have been deleted
|
|
or updated so that it does not meet the query <literal>WHERE</> condition
|
|
anymore, in which case it will not be returned.
|
|
</para>
|
|
</caution>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-TABLE">
|
|
<title><literal>TABLE</literal> Command</title>
|
|
|
|
<para>
|
|
The command
|
|
<programlisting>
|
|
TABLE <replaceable class="parameter">name</replaceable>
|
|
</programlisting>
|
|
is completely equivalent to
|
|
<programlisting>
|
|
SELECT * FROM <replaceable class="parameter">name</replaceable>
|
|
</programlisting>
|
|
It can be used as a top-level command or as a space-saving syntax
|
|
variant in parts of complex queries.
|
|
</para>
|
|
</refsect2>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Examples</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
|
|
...
|
|
</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 hours';
|
|
|
|
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>
|
|
The next 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 the letter W in each table. Only distinct rows are wanted, so the
|
|
key word <literal>ALL</literal> 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>
|
|
|
|
<para>
|
|
This example shows how to use a function in the <literal>FROM</>
|
|
clause, both with and without a column definition list:
|
|
|
|
<programlisting>
|
|
CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS $$
|
|
SELECT * FROM distributors WHERE did = $1;
|
|
$$ LANGUAGE SQL;
|
|
|
|
SELECT * FROM distributors(111);
|
|
did | name
|
|
-----+-------------
|
|
111 | Walt Disney
|
|
|
|
CREATE FUNCTION distributors_2(int) RETURNS SETOF record AS $$
|
|
SELECT * FROM distributors WHERE did = $1;
|
|
$$ LANGUAGE SQL;
|
|
|
|
SELECT * FROM distributors_2(111) AS (f1 int, f2 text);
|
|
f1 | f2
|
|
-----+-------------
|
|
111 | Walt Disney
|
|
</programlisting>
|
|
</para>
|
|
|
|
<para>
|
|
This example shows how to use a simple <literal>WITH</> clause:
|
|
|
|
<programlisting>
|
|
WITH t AS (
|
|
SELECT random() as x FROM generate_series(1, 3)
|
|
)
|
|
SELECT * FROM t
|
|
UNION ALL
|
|
SELECT * FROM t
|
|
|
|
x
|
|
--------------------
|
|
0.534150459803641
|
|
0.520092216785997
|
|
0.0735620250925422
|
|
0.534150459803641
|
|
0.520092216785997
|
|
0.0735620250925422
|
|
</programlisting>
|
|
|
|
Notice that the <literal>WITH</> query was evaluated only once,
|
|
so that we got two sets of the same three random values.
|
|
</para>
|
|
|
|
<para>
|
|
This example uses <literal>WITH RECURSIVE</literal> to find all
|
|
subordinates (direct or indirect) of the employee Mary, and their
|
|
level of indirectness, from a table that shows only direct
|
|
subordinates:
|
|
|
|
<programlisting>
|
|
WITH RECURSIVE employee_recursive(distance, employee_name, manager_name) AS (
|
|
SELECT 1, employee_name, manager_name
|
|
FROM employee
|
|
WHERE manager_name = 'Mary'
|
|
UNION ALL
|
|
SELECT er.distance + 1, e.employee_name, e.manager_name
|
|
FROM employee_recursive er, employee e
|
|
WHERE er.employee_name = e.manager_name
|
|
)
|
|
SELECT distance, employee_name FROM employee_recursive;
|
|
</programlisting>
|
|
|
|
Notice the typical form of recursive queries:
|
|
an initial condition, followed by <literal>UNION</literal>,
|
|
followed by the recursive part of the query. Be sure that the
|
|
recursive part of the query will eventually return no tuples, or
|
|
else the query will loop indefinitely. (See <xref linkend="queries-with">
|
|
for more examples.)
|
|
</para>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Compatibility</title>
|
|
|
|
<para>
|
|
Of course, the <command>SELECT</command> statement is compatible
|
|
with the SQL standard. But there are some extensions and some
|
|
missing features.
|
|
</para>
|
|
|
|
<refsect2>
|
|
<title>Omitted <literal>FROM</literal> Clauses</title>
|
|
|
|
<para>
|
|
<productname>PostgreSQL</productname> allows one to omit the
|
|
<literal>FROM</literal> clause. It has a straightforward use to
|
|
compute the results of simple expressions:
|
|
<programlisting>
|
|
SELECT 2+2;
|
|
|
|
?column?
|
|
----------
|
|
4
|
|
</programlisting>
|
|
Some other <acronym>SQL</acronym> databases cannot do this except
|
|
by introducing a dummy one-row table from which to do the
|
|
<command>SELECT</command>.
|
|
</para>
|
|
|
|
<para>
|
|
Note that if a <literal>FROM</literal> clause is not specified,
|
|
the query cannot reference any database tables. For example, the
|
|
following query is invalid:
|
|
<programlisting>
|
|
SELECT distributors.* WHERE distributors.name = 'Westward';
|
|
</programlisting>
|
|
<productname>PostgreSQL</productname> releases prior to
|
|
8.1 would accept queries of this form, and add an implicit entry
|
|
to the query's <literal>FROM</literal> clause for each table
|
|
referenced by the query. This is no longer the default behavior,
|
|
because it does not comply with the SQL standard, and is
|
|
considered by many to be error-prone. For compatibility with
|
|
applications that rely on this behavior the <xref
|
|
linkend="guc-add-missing-from"> configuration variable can be
|
|
enabled.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Omitting the <literal>AS</literal> Key Word</title>
|
|
|
|
<para>
|
|
In the SQL standard, the optional key word <literal>AS</> can be
|
|
omitted before an output column name whenever the new column name
|
|
is a valid column name (that is, not the same as any reserved
|
|
keyword). <productname>PostgreSQL</productname> is slightly more
|
|
restrictive: <literal>AS</> is required if the new column name
|
|
matches any keyword at all, reserved or not. Recommended practice is
|
|
to use <literal>AS</> or double-quote output column names, to prevent
|
|
any possible conflict against future keyword additions.
|
|
</para>
|
|
|
|
<para>
|
|
In <literal>FROM</literal> items, both the standard and
|
|
<productname>PostgreSQL</productname> allow <literal>AS</> to
|
|
be omitted before an alias that is an unreserved keyword. But
|
|
this is impractical for output column names, because of syntactic
|
|
ambiguities.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Namespace Available to <literal>GROUP BY</literal> and <literal>ORDER BY</literal></title>
|
|
|
|
<para>
|
|
In the SQL-92 standard, an <literal>ORDER BY</literal> clause can
|
|
only use output column names or numbers, while a <literal>GROUP
|
|
BY</literal> clause can only use expressions based on input column
|
|
names. <productname>PostgreSQL</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>PostgreSQL</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
|
|
output-column names.
|
|
</para>
|
|
|
|
<para>
|
|
SQL:1999 and later use a slightly different definition which is not
|
|
entirely upward compatible with SQL-92.
|
|
In most cases, however, <productname>PostgreSQL</productname>
|
|
will interpret an <literal>ORDER BY</literal> or <literal>GROUP
|
|
BY</literal> expression the same way SQL:1999 does.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title><literal>WINDOW</literal> Clause Restrictions</title>
|
|
|
|
<para>
|
|
The SQL standard provides additional options for the window
|
|
<replaceable class="parameter">frame_clause</>.
|
|
<productname>PostgreSQL</productname> currently supports only the
|
|
options listed above.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title><literal>LIMIT</literal> and <literal>OFFSET</literal></title>
|
|
|
|
<para>
|
|
The clauses <literal>LIMIT</literal> and <literal>OFFSET</literal>
|
|
are <productname>PostgreSQL</productname>-specific syntax, also
|
|
used by <productname>MySQL</productname>. The SQL:2008 standard
|
|
has introduced the clauses <literal>OFFSET ... FETCH {FIRST|NEXT}
|
|
...</literal> for the same functionality, as shown above
|
|
in <xref linkend="sql-limit" endterm="sql-limit-title">, and this
|
|
syntax is also used by <productname>IBM DB2</productname>.
|
|
(Applications written for <productname>Oracle</productname>
|
|
frequently use a workaround involving the automatically
|
|
generated <literal>rownum</literal> column, not available in
|
|
PostgreSQL, to implement the effects of these clauses.)
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>Nonstandard Clauses</title>
|
|
|
|
<para>
|
|
The clause <literal>DISTINCT ON</literal> is not defined in the
|
|
SQL standard.
|
|
</para>
|
|
</refsect2>
|
|
</refsect1>
|
|
</refentry>
|