doc/src/sgml/ref/select.sgml

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<refentry id="SQL-SELECT">

 <refmeta>

  <refentrytitle id="sql-select-title">SELECT</refentrytitle>

  <refmiscinfo>SQL - Language Statements</refmiscinfo>

 </refmeta>



 <refnamediv>

  <refname>SELECT</refname>

  <refpurpose>retrieve rows from a table or view</refpurpose>

 </refnamediv>



 <indexterm zone="sql-select">

  <primary>SELECT</primary>

 </indexterm>



 <refsynopsisdiv>

<synopsis>

[ WITH <replaceable class="parameter">with_query</replaceable> [, ...] ]
SELECT [ ALL | DISTINCT [ ON ( <replaceable class="parameter">expression</replaceable> [, ...] ) ] ]

    * | <replaceable class="parameter">expression</replaceable> [ [AS] <replaceable class="parameter">output_name</replaceable> ] [, ...]

    [ FROM <replaceable class="parameter">from_item</replaceable> [, ...] ]

    [ WHERE <replaceable class="parameter">condition</replaceable> ]

    [ GROUP BY <replaceable class="parameter">grouping_element</replaceable> [, ...] ]

    [ HAVING <replaceable class="parameter">condition</replaceable> [, ...] ]

	[ WINDOW <replaceable class="parameter">window_name</replaceable> AS (<replaceable class="parameter">window_specification</replaceable>) ]

    [ { UNION | INTERSECT | EXCEPT } [ ALL ] <replaceable class="parameter">select</replaceable> ]

    [ ORDER BY <replaceable class="parameter">expression</replaceable> [ ASC | DESC | USING <replaceable class="parameter">operator</replaceable> ] [, ...] ]

    [ LIMIT { <replaceable class="parameter">count</replaceable> | ALL } ]

    [ OFFSET <replaceable class="parameter">start</replaceable> ]

    [ FOR { UPDATE | SHARE } [ OF <replaceable class="parameter">table_name</replaceable> [, ...] ] [ NOWAIT ] [...] ]

	

where <replaceable class="parameter">grouping_element</replaceable> can be one of:

    ()

    <replaceable class="parameter">expression</replaceable>

    ROLLUP ( [ <replaceable class="parameter">expression</replaceable> [, ...] ] )

    CUBE ( [ <replaceable class="parameter">expression</replaceable> [, ...] ] )

    GROUPING SETS ( ( <replaceable class="parameter">grouping_element</replaceable> [, ...] ) ) 

	



where <replaceable class="parameter">window_specification</replaceable> can be:

    [ <replaceable class="parameter">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> ] [, ...] ]

    [ {RANGE | ROWS} 

           { UNBOUNDED PRECEDING

           | <replaceable class="parameter">expression</replaceable> PRECEDING

           | CURRENT ROW

           | BETWEEN <replaceable class="parameter">window_frame_bound</replaceable> AND <replaceable class="parameter">window_frame_bound</replaceable> } ]

     where <replaceable class="parameter">window_frame_bound</replaceable> can be one of:

           UNBOUNDED PRECEDING

           <replaceable class="parameter">expression</replaceable> PRECEDING

           CURRENT ROW

           <replaceable class="parameter">expression</replaceable> FOLLOWING

           UNBOUNDED FOLLOWING



where <replaceable class="parameter">from_item</replaceable> can be one of:



    [ ONLY ] <replaceable class="parameter">table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ] ]

    ( <replaceable class="parameter">select</replaceable> ) [ AS ] <replaceable class="parameter">alias</replaceable> [ ( <replaceable class="parameter">column_alias</replaceable> [, ...] ) ]

    <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> [, ...] ) ]

    <replaceable class="parameter">function_name</replaceable> ( [ <replaceable class="parameter">argument</replaceable> [, ...] ] ) AS ( <replaceable class="parameter">column_definition</replaceable> [, ...] )

    <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> [, ...] ) ]



where <replaceable class="parameter">group_elem</replaceable> can be one of:

    <replaceable class="parameter">expression</replaceable> [, ...] ]

    ROLLUP ( <replaceable class="parameter">expression</replaceable> [, ...] ] )

    CUBE ( <replaceable class="parameter">expression</replaceable> [, ...] ] )

    GROUPING SETS ( <replaceable class="parameter">group_elem</replaceable> [, ...] ] )

    ( )


and <replaceable class="parameter">with_query</replaceable> can be:

    <replaceable class="parameter">query_name</replaceable> [ ( <replaceable class="parameter">column</replaceable> [, ...] ) ]
    AS ( { VALUES ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) | <replaceable class="parameter">query</replaceable> } )


</synopsis>



 </refsynopsisdiv>



 <refsect1>

  <title>Description</title>



  <para>

   <command>SELECT</command> retrieves rows from zero or more tables.

   The general processing of <command>SELECT</command> is as follows:



   <orderedlist>

    <listitem>

     <para>

      All elements in the <literal>FROM</literal> list are computed.

      (Each element in the <literal>FROM</literal> list is a real or

      virtual table.)  If more than one element is specified in the

      <literal>FROM</literal> list, they are cross-joined together.

      (See <xref linkend="sql-from" endterm="sql-from-title"> below.)

     </para>

    </listitem>



    <listitem>

     <para>

      If the <literal>WHERE</literal> clause is specified, all rows

      that do not satisfy the condition are eliminated from the

      output.  (See <xref linkend="sql-where"

      endterm="sql-where-title"> below.)

     </para>

    </listitem>



    <listitem>

     <para>

      If the <literal>GROUP BY</literal> clause is specified, the

      output is divided into groups of rows that match on one or more

      values.  If the <literal>HAVING</literal> clause is present, it

      eliminates groups that do not satisfy the given condition.  (See

      <xref linkend="sql-groupby" endterm="sql-groupby-title"> and

      <xref linkend="sql-having" endterm="sql-having-title"> below.)

     </para>

    </listitem>



    <listitem>

     <para>

      The actual output rows are computed using the

      <command>SELECT</command> output expressions for each selected

      row.  (See

      <xref linkend="sql-select-list" endterm="sql-select-list-title">

      below.)

     </para>

    </listitem>



    <listitem>

     <para>

      Using the operators <literal>UNION</literal>,

      <literal>INTERSECT</literal>, and <literal>EXCEPT</literal>, the

      output of more than one <command>SELECT</command> statement can

      be combined to form a single result set.  The

      <literal>UNION</literal> operator returns all rows that are in

      one or both of the result sets.  The

      <literal>INTERSECT</literal> operator returns all rows that are

      strictly in both result sets.  The <literal>EXCEPT</literal>

      operator returns the rows that are in the first result set but

      not in the second.  In all three cases, duplicate rows are

      eliminated unless <literal>ALL</literal> is specified. (See

      <xref linkend="sql-union" endterm="sql-union-title">, <xref

      linkend="sql-intersect" endterm="sql-intersect-title">, and

      <xref linkend="sql-except" endterm="sql-except-title"> below.)

     </para>

    </listitem>



    <listitem>

     <para>

      If the <literal>ORDER BY</literal> clause is specified, the

      returned rows are sorted in the specified order.  If

      <literal>ORDER BY</literal> is not given, the rows are returned

      in whatever order the system finds fastest to produce.  (See

      <xref linkend="sql-orderby" endterm="sql-orderby-title"> below.)

     </para>

    </listitem>



    <listitem>

     <para>

      <literal>DISTINCT</literal> eliminates duplicate rows from the

      result.  <literal>DISTINCT ON</literal> eliminates rows that

      match on all the specified expressions.  <literal>ALL</literal>

      (the default) will return all candidate rows, including

      duplicates.  (See <xref linkend="sql-distinct"

      endterm="sql-distinct-title"> below.)

     </para>

    </listitem>



    <listitem>

     <para>

      If the <literal>LIMIT</literal> or <literal>OFFSET</literal>

      clause is specified, the <command>SELECT</command> statement

      only returns a subset of the result rows. (See <xref

      linkend="sql-limit" endterm="sql-limit-title"> below.)

     </para>

    </listitem>



    <listitem>

     <para>

      If <literal>FOR UPDATE</literal> or <literal>FOR SHARE</literal>

      is specified, the

      <command>SELECT</command> statement locks the selected rows

      against concurrent updates.  (See <xref linkend="sql-for-update-share"

      endterm="sql-for-update-share-title"> below.)

     </para>

    </listitem>

   </orderedlist>

  </para>



  <para>

   You must have <literal>SELECT</literal> privilege on a table to

   read its values.  The use of <literal>FOR UPDATE</literal> or

   <literal>FOR SHARE</literal> requires

   <literal>UPDATE</literal> privilege as well.

  </para>

 </refsect1>



 <refsect1>

  <title>Parameters</title>



  <refsect2 id="SQL-FROM">

   <title id="sql-from-title"><literal>FROM</literal> Clause</title>



   <para>

    The <literal>FROM</literal> clause specifies one or more source

    tables for the <command>SELECT</command>.  If multiple sources are

    specified, the result is the Cartesian product (cross join) of all

    the sources.  But usually qualification conditions

    are added to restrict the returned rows to a small subset of the

    Cartesian product.

   </para>



   <para>

    The <literal>FROM</literal> clause can contain the following

    elements:



    <variablelist>

     <varlistentry>

      <term><replaceable class="parameter">table_name</replaceable></term>

      <listitem>

       <para>

        The name (optionally schema-qualified) of an existing table or

        view.  If <literal>ONLY</> is specified, only that table is

        scanned.  If <literal>ONLY</> is not specified, the table and

        all its descendant tables (if any) are scanned.  <literal>*</>

        can be appended to the table name to indicate that descendant

        tables are to be scanned, but in the current version, this is

        the default behavior.  (In releases before 7.1,

        <literal>ONLY</> was the default behavior.)  The default

        behavior can be modified by changing the <xref

        linkend="guc-sql-inheritance"> configuration option.

       </para>

      </listitem>

     </varlistentry>

     

     <varlistentry>

      <term><replaceable class="parameter">alias</replaceable></term>

      <listitem>

       <para>

        A substitute name for the <literal>FROM</> item containing the

        alias.  An alias is used for brevity or to eliminate ambiguity

        for self-joins (where the same table is scanned multiple

        times).  When an alias is provided, it completely hides the

        actual name of the table or function; for example given

        <literal>FROM foo AS f</>, the remainder of the

        <command>SELECT</command> must refer to this <literal>FROM</>

        item as <literal>f</> not <literal>foo</>.  If an alias is

        written, a column alias list can also be written to provide

        substitute names for one or more columns of the table.

       </para>

      </listitem>

     </varlistentry>

     

     <varlistentry>

      <term><replaceable class="parameter">select</replaceable></term>

      <listitem>

       <para>

        A sub-<command>SELECT</command> can appear in the

        <literal>FROM</literal> clause.  This acts as though its

        output were created as a temporary table for the duration of

        this single <command>SELECT</command> command.  Note that the

        sub-<command>SELECT</command> must be surrounded by

        parentheses, and an alias <emphasis>must</emphasis> be

        provided for it.  A

        <xref linkend="sql-values" endterm="sql-values-title"> command

        can also be used here.

       </para>

      </listitem>

     </varlistentry>



     <varlistentry>

      <term><replaceable class="parameter">function_name</replaceable></term>

      <listitem>

       <para>

        Function calls can appear in the <literal>FROM</literal>

        clause.  (This is especially useful for functions that return

        result sets, but any function can be used.)  This acts as

        though its output were created as a temporary table for the

        duration of this single <command>SELECT</command> command. An

        alias may also be used. If an alias is written, a column alias

        list can also be written to provide substitute names for one

        or more attributes of the function's composite return type. If

        the function has been defined as returning the <type>record</>

        data type, then an alias or the key word <literal>AS</> must

        be present, followed by a column definition list in the form

        <literal>( <replaceable

        class="parameter">column_name</replaceable> <replaceable

        class="parameter">data_type</replaceable> <optional>, ... </>

        )</literal>.  The column definition list must match the actual

        number and types of columns returned by the function.

       </para>

      </listitem>

     </varlistentry>

     

     <varlistentry>

      <term><replaceable class="parameter">join_type</replaceable></term>

      <listitem>

       <para>

        One of

        <itemizedlist>

         <listitem>

          <para><literal>[ INNER ] JOIN</literal></para>

         </listitem>

         <listitem>

          <para><literal>LEFT [ OUTER ] JOIN</literal></para>

         </listitem>

         <listitem>

          <para><literal>RIGHT [ OUTER ] JOIN</literal></para>

         </listitem>

         <listitem>

          <para><literal>FULL [ OUTER ] JOIN</literal></para>

         </listitem>

         <listitem>

          <para><literal>CROSS JOIN</literal></para>

         </listitem>

        </itemizedlist>



        For the <literal>INNER</> and <literal>OUTER</> join types, a

        join condition must be specified, namely exactly one of

        <literal>NATURAL</>, <literal>ON <replaceable

        class="parameter">join_condition</replaceable></literal>, or

        <literal>USING (<replaceable

        class="parameter">join_column</replaceable> [, ...])</literal>.

        See below for the meaning.  For <literal>CROSS JOIN</literal>,

        none of these clauses may appear.

       </para>



       <para>

        A <literal>JOIN</literal> clause combines two

        <literal>FROM</> items.  Use parentheses if necessary to

        determine the order of nesting.  In the absence of parentheses,

        <literal>JOIN</literal>s nest left-to-right.  In any case

        <literal>JOIN</literal> binds more tightly than the commas

        separating <literal>FROM</> items.

       </para>



       <para>

        <literal>CROSS JOIN</> and <literal>INNER JOIN</literal>

        produce a simple Cartesian product, the same result as you get from

        listing the two items at the top level of <literal>FROM</>,

        but restricted by the join condition (if any).

        <literal>CROSS JOIN</> is equivalent to <literal>INNER JOIN ON

        (TRUE)</>, that is, no rows are removed by qualification.

        These join types are just a notational convenience, since they

        do nothing you couldn't do with plain <literal>FROM</> and

        <literal>WHERE</>.

       </para>



       <para>

        <literal>LEFT OUTER JOIN</> returns all rows in the qualified

        Cartesian product (i.e., all combined rows that pass its join

        condition), plus one copy of each row in the left-hand table

        for which there was no right-hand row that passed the join

        condition.  This left-hand row is extended to the full width

        of the joined table by inserting null values for the

        right-hand columns.  Note that only the <literal>JOIN</>

        clause's own condition is considered while deciding which rows

        have matches.  Outer conditions are applied afterwards.

       </para>



       <para>

        Conversely, <literal>RIGHT OUTER JOIN</> returns all the

        joined rows, plus one row for each unmatched right-hand row

        (extended with nulls on the left).  This is just a notational

        convenience, since you could convert it to a <literal>LEFT

        OUTER JOIN</> by switching the left and right inputs.

       </para>



       <para>

        <literal>FULL OUTER JOIN</> returns all the joined rows, plus

        one row for each unmatched left-hand row (extended with nulls

        on the right), plus one row for each unmatched right-hand row

        (extended with nulls on the left).

       </para>

      </listitem>

     </varlistentry>

     

     <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-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</>, <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</> 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</>, <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</> 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</>, <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</> 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 &lt;</> and

    <literal>DESC</> is usually equivalent to <literal>USING &gt;</>.

    (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 &mdash; 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-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) &mdash; 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 can't be clearly identified with

    individual table rows; for example they can't 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>

 </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) &lt; 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>

    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>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 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>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>