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1144 lines
44 KiB
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1144 lines
44 KiB
Plaintext
<!--
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$PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.80.4.3 2006/12/01 20:50:06 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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<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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<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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<refsynopsisdiv>
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<synopsis>
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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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[ { 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> ] [, ...] ]
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[ LIMIT { <replaceable class="parameter">count</replaceable> | ALL } ]
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[ OFFSET <replaceable class="parameter">start</replaceable> ]
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[ FOR UPDATE [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] ]
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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">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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</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 one 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 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>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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The <literal>FOR UPDATE</literal> clause causes the
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<command>SELECT</command> statement to lock the selected rows
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against concurrent updates. (See <xref linkend="sql-for-update"
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endterm="sql-for-update-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> 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-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.
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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 may 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
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<literal>USING (<replaceable
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class="parameter">join_column</replaceable> [, ...])</literal>.
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See below for the meaning. For <literal>CROSS JOIN</literal>,
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none of these clauses may 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</>.
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</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
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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>
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<term><literal>ON <replaceable class="parameter">join_condition</replaceable></literal></term>
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<listitem>
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<para>
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<replaceable class="parameter">join_condition</replaceable> is
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an expression resulting in a value of type
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<type>boolean</type> (similar to a <literal>WHERE</literal>
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clause) that specifies which rows in a join are considered to
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match.
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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><literal>USING (<replaceable class="parameter">join_column</replaceable> [, ...])</literal></term>
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<listitem>
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<para>
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A clause of the form <literal>USING ( a, b, ... )</literal> is
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shorthand for <literal>ON left_table.a = right_table.a AND
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left_table.b = right_table.b ...</literal>. Also,
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<literal>USING</> implies that only one of each pair of
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equivalent columns will be included in the join output, not
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both.
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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><literal>NATURAL</literal></term>
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<listitem>
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<para>
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<literal>NATURAL</literal> is shorthand for a
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<literal>USING</> list that mentions all columns in the two
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tables that have the same names.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</para>
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</refsect2>
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<refsect2 id="SQL-WHERE">
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<title id="sql-where-title"><literal>WHERE</literal> Clause</title>
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<para>
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The optional <literal>WHERE</literal> clause has the general form
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<synopsis>
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WHERE <replaceable class="parameter">condition</replaceable>
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</synopsis>
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where <replaceable class="parameter">condition</replaceable> is
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any expression that evaluates to a result of type
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<type>boolean</type>. Any row that does not satisfy this
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condition will be eliminated from the output. A row satisfies the
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condition if it returns true when the actual row values are
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substituted for any variable references.
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</para>
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</refsect2>
|
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|
|
<refsect2 id="SQL-GROUPBY">
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<title id="sql-groupby-title"><literal>GROUP BY</literal> Clause</title>
|
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|
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<para>
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The optional <literal>GROUP BY</literal> clause has the general form
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<synopsis>
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GROUP BY <replaceable class="parameter">expression</replaceable> [, ...]
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</synopsis>
|
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</para>
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|
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<para>
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<literal>GROUP BY</literal> will condense into a single row all
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selected rows that share the same values for the grouped
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expressions. <replaceable
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class="parameter">expression</replaceable> can be an input column
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name, or the name or ordinal number of an output column
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(<command>SELECT</command> list item), or an arbitrary
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|
expression formed from input-column values. In case of ambiguity,
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a <literal>GROUP BY</literal> name will be interpreted as an
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input-column name rather than an output column name.
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</para>
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<para>
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Aggregate functions, if any are used, are computed across all rows
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making up each group, producing a separate value for each group
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(whereas without <literal>GROUP BY</literal>, an aggregate
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|
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.
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|
</para>
|
|
</refsect2>
|
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|
|
<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
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<synopsis>
|
|
HAVING <replaceable class="parameter">condition</replaceable>
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|
</synopsis>
|
|
where <replaceable class="parameter">condition</replaceable> is
|
|
the same as specified for the <literal>WHERE</literal> clause.
|
|
</para>
|
|
|
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<para>
|
|
<literal>HAVING</literal> eliminates group rows that do not
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satisfy the condition. <literal>HAVING</literal> is different
|
|
from <literal>WHERE</literal>: <literal>WHERE</literal> filters
|
|
individual rows before the application of <literal>GROUP
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BY</literal>, while <literal>HAVING</literal> filters group rows
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|
created by <literal>GROUP BY</literal>. Each column referenced in
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|
<replaceable class="parameter">condition</replaceable> must
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unambiguously reference a grouping column, unless the reference
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appears within an aggregate function.
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|
</para>
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|
</refsect2>
|
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|
|
<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. Using the clause
|
|
<literal>AS <replaceable
|
|
class="parameter">output_name</replaceable></literal>, another
|
|
name can be specified for an output column. This name is
|
|
primarily used to label the column for display. It can also 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, one can write <literal><replaceable
|
|
class="parameter">table_name</replaceable>.*</literal> as a
|
|
shorthand for the columns coming from just that table.
|
|
</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</>, or <literal>FOR UPDATE</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</> may not 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</>, or <literal>FOR UPDATE</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</> may not 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</>, or <literal>FOR UPDATE</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</> may not 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> ] [, ...]
|
|
</synopsis>
|
|
<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 <literal>ORDER BY</literal> clause causes the result rows to
|
|
be sorted according to the specified expressions. If two rows are
|
|
equal according to the leftmost expression, the 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>
|
|
The ordinal number refers to the ordinal (left-to-right) position
|
|
of the result column. This feature makes it possible to define an
|
|
ordering on the basis of a column that does not have a unique
|
|
name. This is never absolutely necessary because it is always
|
|
possible to assign a name to a result 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> result 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 may 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 a result column name and an input column name,
|
|
<literal>ORDER BY</> will interpret it as the result 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 may 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 may be specified in the <literal>USING</> clause.
|
|
<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>
|
|
The null value sorts higher than any other value. In other words,
|
|
with ascending sort order, null values sort at the end, and with
|
|
descending sort order, null values sort at the beginning.
|
|
</para>
|
|
|
|
<para>
|
|
Character-string data is sorted according to the locale-specific
|
|
collation order that was established when the database cluster
|
|
was initialized.
|
|
</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>
|
|
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 may 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>
|
|
</refsect2>
|
|
|
|
<refsect2 id="SQL-FOR-UPDATE">
|
|
<title id="sql-for-update-title"><literal>FOR UPDATE</literal> Clause</title>
|
|
|
|
<para>
|
|
The <literal>FOR UPDATE</literal> clause has this form:
|
|
<synopsis>
|
|
FOR UPDATE [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ]
|
|
</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>
|
|
If specific tables are named in <literal>FOR UPDATE</literal>,
|
|
then only rows coming from those tables are locked; any other
|
|
tables used in the <command>SELECT</command> are simply read as
|
|
usual.
|
|
</para>
|
|
|
|
<para>
|
|
<literal>FOR UPDATE</literal> cannot be used in contexts where
|
|
returned rows can't be clearly identified with individual table
|
|
rows; for example it can't be used with aggregation.
|
|
</para>
|
|
|
|
<para>
|
|
<literal>FOR UPDATE</literal> may appear before
|
|
<literal>LIMIT</literal> for compatibility with
|
|
<productname>PostgreSQL</productname> versions before 7.3. It
|
|
effectively executes after <literal>LIMIT</literal>, however, and
|
|
so that is the recommended place to write it.
|
|
</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: 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</literal>
|
|
clauses to return fewer rows than specified by <literal>LIMIT</literal>.
|
|
This is because <literal>LIMIT</> selects a number of rows,
|
|
but might then block requesting a <literal>FOR UPDATE</literal> lock.
|
|
Once the <literal>SELECT</> unblocks, the query qualification might not
|
|
be met and the row not be returned by <literal>SELECT</>.
|
|
</para>
|
|
</caution>
|
|
</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>
|
|
</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>
|
|
A less obvious use is to abbreviate a normal
|
|
<command>SELECT</command> from tables:
|
|
<programlisting>
|
|
SELECT distributors.* WHERE distributors.name = 'Westward';
|
|
|
|
did | name
|
|
-----+----------
|
|
108 | Westward
|
|
</programlisting>
|
|
This works because an implicit <literal>FROM</literal> item is
|
|
added for each table that is referenced in other parts of the
|
|
<command>SELECT</command> statement but not mentioned in
|
|
<literal>FROM</literal>.
|
|
</para>
|
|
|
|
<para>
|
|
While this is a convenient shorthand, it's easy to misuse. For
|
|
example, the command
|
|
<programlisting>
|
|
SELECT distributors.* FROM distributors d;
|
|
</programlisting>
|
|
is probably a mistake; most likely the user meant
|
|
<programlisting>
|
|
SELECT d.* FROM distributors d;
|
|
</programlisting>
|
|
rather than the unconstrained join
|
|
<programlisting>
|
|
SELECT distributors.* FROM distributors d, distributors distributors;
|
|
</programlisting>
|
|
that he will actually get. To help detect this sort of mistake,
|
|
<productname>PostgreSQL</productname> will warn if the
|
|
implicit-<literal>FROM</literal> feature is used in a
|
|
<command>SELECT</command> statement that also contains an explicit
|
|
<literal>FROM</literal> clause. Also, it is possible to disable
|
|
the implicit-<literal>FROM</literal> feature by setting the
|
|
<xref linkend="guc-add-missing-from"> parameter to false.
|
|
</para>
|
|
</refsect2>
|
|
|
|
<refsect2>
|
|
<title>The <literal>AS</literal> Key Word</title>
|
|
|
|
<para>
|
|
In the SQL standard, the optional key word <literal>AS</> is just
|
|
noise and can be omitted without affecting the meaning. The
|
|
<productname>PostgreSQL</productname> parser requires this key
|
|
word when renaming output columns because the type extensibility
|
|
features lead to parsing ambiguities without it.
|
|
<literal>AS</literal> is optional in <literal>FROM</literal>
|
|
items, however.
|
|
</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 may
|
|
only use result column names or numbers, while a <literal>GROUP
|
|
BY</literal> clause may 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
|
|
result-column names.
|
|
</para>
|
|
|
|
<para>
|
|
SQL:1999 uses 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>Nonstandard Clauses</title>
|
|
|
|
<para>
|
|
The clauses <literal>DISTINCT ON</literal>,
|
|
<literal>LIMIT</literal>, and <literal>OFFSET</literal> are not
|
|
defined in the SQL standard.
|
|
</para>
|
|
</refsect2>
|
|
</refsect1>
|
|
</refentry>
|
|
|
|
<!-- Keep this comment at the end of the file
|
|
Local variables:
|
|
mode: sgml
|
|
sgml-omittag:nil
|
|
sgml-shorttag:t
|
|
sgml-minimize-attributes:nil
|
|
sgml-always-quote-attributes:t
|
|
sgml-indent-step:1
|
|
sgml-indent-data:t
|
|
sgml-parent-document:nil
|
|
sgml-default-dtd-file:"../reference.ced"
|
|
sgml-exposed-tags:nil
|
|
sgml-local-catalogs:"/usr/lib/sgml/catalog"
|
|
sgml-local-ecat-files:nil
|
|
End:
|
|
-->
|