select.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.47 2001/09/23 03:39:01 momjian Exp $
Postgres documentation
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<refentry id="SQL-SELECT">
 <refmeta>
  <refentrytitle id="sql-select-title">SELECT</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   SELECT
  </refname>
  <refpurpose>
   retrieve rows from a table or view
  </refpurpose></refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2000-12-11</date>
  </refsynopsisdivinfo>
  <synopsis>
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">expression</replaceable> [, ...] ]
    [ HAVING <replaceable class="PARAMETER">condition</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> ] [, ...] ]
    [ FOR UPDATE [ OF <replaceable class="PARAMETER">tablename</replaceable> [, ...] ] ]
    [ LIMIT [ <replaceable class="PARAMETER">start</replaceable> , ] { <replaceable class="PARAMETER">count</replaceable> | ALL } ]
    [ OFFSET <replaceable class="PARAMETER">start</replaceable> ]

where <replaceable class="PARAMETER">from_item</replaceable> can be:

[ ONLY ] <replaceable class="PARAMETER">table_name</replaceable> [ * ]
    [ [ AS ] <replaceable class="PARAMETER">alias</replaceable> [ ( <replaceable class="PARAMETER">column_alias_list</replaceable> ) ] ]
|
( <replaceable class="PARAMETER">select</replaceable> )
    [ AS ] <replaceable class="PARAMETER">alias</replaceable> [ ( <replaceable class="PARAMETER">column_alias_list</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_list</replaceable> ) ]
  </synopsis>
  
  <refsect2 id="R2-SQL-SELECT-1">
   <refsect2info>
    <date>2000-12-11</date>
   </refsect2info>
   <title>
    Inputs
   </title>

   <para>
    <variablelist>
     <varlistentry>
      <term><replaceable class="PARAMETER">expression</replaceable></term>
      <listitem>
       <para>
	The name of a table's column or an expression.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">output_name</replaceable></term>
      <listitem>
       <para>
	Specifies another name for an output column using
	the AS clause.  This name is primarily used to label the column
	for display.  It can also be used to refer to the column's value in
	ORDER BY and GROUP BY clauses.  But the
	<replaceable class="PARAMETER">output_name</replaceable>
	cannot be used in the WHERE or HAVING clauses; write out the
	expression instead.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">from_item</replaceable></term>
      <listitem>
       <para>
        A table reference, sub-SELECT, or JOIN clause.  See below for details.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">condition</replaceable></term>
      <listitem>
       <para>
	A boolean expression giving a result of true or false.
	See the WHERE and HAVING clause descriptions below.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">select</replaceable></term>
      <listitem>
       <para>
	A select statement with all features except the ORDER BY, FOR UPDATE,
	and LIMIT clauses (even those can be used when the select is
	parenthesized).
       </para>
      </listitem>
     </varlistentry>
     
    </variablelist>
   </para>

   <para>
    FROM items can contain:
    <variablelist>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">table_name</replaceable></term>
      <listitem>
       <para>
	The name of an existing table or view.  If ONLY is specified, only that
	table is scanned.  If ONLY is not specified, the table and all its
	descendant tables (if any) are scanned.  * 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, ONLY was the default behavior.)
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">alias</replaceable></term>
      <listitem>
       <para>
	A substitute name for the preceding
	<replaceable class="PARAMETER">table_name</replaceable>.
	An alias is used for brevity or to eliminate ambiguity for self-joins
	(where the same table is scanned multiple times).  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-SELECT can appear in the FROM clause.  This acts as though
	its output were created as a temporary table for the duration of
	this single SELECT command.  Note that the sub-SELECT must be
	surrounded by parentheses, and an alias <emphasis>must</emphasis>
	be provided for it.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">join_type</replaceable></term>
      <listitem>
       <para>
       One of
       <command>[ INNER ] JOIN</command>,
       <command>LEFT [ OUTER ] JOIN</command>,
       <command>RIGHT [ OUTER ] JOIN</command>,
       <command>FULL [ OUTER ] JOIN</command>, or
       <command>CROSS JOIN</command>.
       For INNER and OUTER join types, exactly one of NATURAL,
       ON <replaceable class="PARAMETER">join_condition</replaceable>, or
       USING ( <replaceable class="PARAMETER">join_column_list</replaceable> )
       must appear.  For CROSS JOIN, none of these items may appear.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">join_condition</replaceable></term>
      <listitem>
       <para>
       A qualification condition.  This is similar to the WHERE condition
       except that it only applies to the two from_items being joined in
       this JOIN clause.
       </para>
      </listitem>
     </varlistentry>
     
     <varlistentry>
      <term><replaceable class="PARAMETER">join_column_list</replaceable></term>
      <listitem>
       <para>
       A USING column list ( a, b, ... ) is shorthand for the ON condition
       left_table.a = right_table.a AND left_table.b = right_table.b ...
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
   </para>
  </refsect2>
  
  <refsect2 id="R2-SQL-SELECT-2">
   <refsect2info>
    <date>1998-09-24</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

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

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

 <refsect1 id="R1-SQL-SELECT-1">
  <refsect1info>
   <date>2000-12-11</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>SELECT</command> will return rows from one or more tables.
   Candidates for selection are rows which satisfy the WHERE condition;
   if WHERE is omitted, all rows are candidates.
   (See <xref linkend="sql-where" endterm="sql-where-title">.)
  </para>

  <para>
   Actually, the returned rows are not directly the rows produced by the
   FROM/WHERE/GROUP BY/HAVING clauses; rather, the output rows are formed
   by computing the SELECT output expressions for each selected row.
   <command>*</command> can be written in the output list as a shorthand
   for all the columns of the selected rows.  Also, one can write
   <replaceable class="PARAMETER">table_name</replaceable><command>.*</command>
   as a shorthand for the columns coming from just that table.
  </para>

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

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

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

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

  <para>
   You must have SELECT privilege to a table to read its values
   (See the <command>GRANT</command>/<command>REVOKE</command> statements).
  </para>
   
  <refsect2 id="SQL-FROM">
   <refsect2info>
    <date>2000-12-11</date>
   </refsect2info>
   <title id="sql-from-title">
    FROM Clause
   </title>

   <para>
    The FROM clause specifies one or more source tables for the SELECT.
    If multiple sources are specified, the result is conceptually the
    Cartesian product of all the rows in all the sources --- but usually
    qualification conditions are added to restrict the returned rows to
    a small subset of the Cartesian product.
   </para>

   <para>
    When a FROM item is a simple table name, it implicitly includes rows
    from sub-tables (inheritance children) of the table.
    <command>ONLY</command> will
    suppress rows from sub-tables of the table.  Before
    <Productname>Postgres</Productname> 7.1,
    this was the default result, and adding sub-tables was done
    by appending <command>*</command> to the table name.
    This old behaviour is available via the command 
    <command>SET SQL_Inheritance TO OFF;</command>
   </para>

   <para>
    A FROM item can also be a parenthesized sub-SELECT (note that an
    alias clause is required for a sub-SELECT!).  This is an extremely
    handy feature since it's the only way to get multiple levels of
    grouping, aggregation, or sorting in a single query.
   </para>

   <para>
    Finally, a FROM item can be a JOIN clause, which combines two simpler
    FROM items.  (Use parentheses if necessary to determine the order
    of nesting.)
   </para>

   <para>
    A CROSS JOIN or INNER JOIN is a simple Cartesian product,
    the same as you get from listing the two items at the top level of FROM.
    CROSS JOIN is equivalent to 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 FROM and
    WHERE.
   </para>

   <para>
    LEFT OUTER JOIN returns all rows in the qualified Cartesian product
    (i.e., all combined rows that pass its ON condition), plus one copy of each
    row in the left-hand table for which there was no right-hand row that
    passed the ON condition.  This left-hand row is extended to the full
    width of the joined table by inserting NULLs for the right-hand columns.
    Note that only the JOIN's own ON or USING condition is considered while
    deciding which rows have matches.  Outer ON or WHERE conditions are
    applied afterwards.
   </para>

   <para>
    Conversely, 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 LEFT OUTER JOIN by switching
    the left and right inputs.
   </para>

   <para>
    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>

   <para>
    For all the JOIN types except CROSS JOIN, you must write exactly one of
    ON <replaceable class="PARAMETER">join_condition</replaceable>,
    USING ( <replaceable class="PARAMETER">join_column_list</replaceable> ),
    or NATURAL.  ON is the most general case: you can write any qualification
    expression involving the two tables to be joined.
    A USING column list ( a, b, ... ) is shorthand for the ON condition
    left_table.a = right_table.a AND left_table.b = right_table.b ...
    Also, USING implies that only one of each pair of equivalent columns will
    be included in the JOIN output, not both.  NATURAL is shorthand for
    a USING list that mentions all similarly-named columns in the tables.
   </para>
  </refsect2>
   
  <refsect2 id="SQL-WHERE">
   <refsect2info>
    <date>2000-03-15</date>
   </refsect2info>
   <title id="sql-where-title">
    WHERE Clause
   </title>

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

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

    or

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

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

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

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

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

   <para>
    Each column referenced in 
    <replaceable class="PARAMETER">boolean_expr</replaceable> shall unambiguously
    reference a grouping column, unless the reference appears within an
    aggregate function.
   </para>
  </refsect2>
  
  <refsect2 id="SQL-ORDERBY">
   <refsect2info>
    <date>2000-03-15</date>
   </refsect2info>
   <title id="sql-orderby-title">
    ORDER BY Clause
   </title>
   <para>
    <synopsis>
ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...]
    </synopsis></para>
    
   <para>
    An ORDER BY item can be the name or ordinal
    number of an output column (SELECT expression), or it can be an arbitrary
    expression formed from input-column values.  In case of ambiguity, an
    ORDER BY name will be interpreted as an output-column name.
   </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 proper name.
    This is never absolutely necessary because it is always possible
    to assign a name to a result column using the AS clause, e.g.:
    <programlisting>
SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
    </programlisting></para>
    
   <para>
    It is also possible to ORDER BY
    arbitrary expressions (an extension to SQL92),
    including fields that do not appear in the
    SELECT result list.
    Thus the following statement is legal:
    <programlisting>
SELECT name FROM distributors ORDER BY code;
    </programlisting>
    A limitation of this feature is that an ORDER BY clause applying to the
    result of a UNION, INTERSECT, or EXCEPT query may only specify an output
    column name or number, not an expression.
   </para>
    
   <para>
    Note that if an ORDER BY item is a simple name that matches both
    a result column name and an input column name, ORDER BY will interpret
    it as the result column name.  This is the opposite of the choice that
    GROUP BY will make in the same situation.  This inconsistency is
    mandated by the SQL92 standard.
   </para>
    
   <para>
    Optionally one may add the keyword DESC (descending)
    or ASC (ascending) after each column name in the ORDER BY clause.
    If not specified, ASC is assumed by default.  Alternatively, a
    specific ordering operator name may be specified.  ASC is equivalent
    to USING &lt; and DESC is equivalent to USING &gt;.
   </para>
  </refsect2>
  
  <refsect2 id="SQL-UNION">
   <refsect2info>
    <date>2000-12-11</date>
   </refsect2info>
   <title id="sql-union-title">
    UNION Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> UNION [ ALL ] <replaceable class="PARAMETER">table_query</replaceable>
    [ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
    [ LIMIT [ <replaceable class="PARAMETER">start</replaceable> , ] { <replaceable class="PARAMETER">count</replaceable> | ALL } ]
    [ OFFSET <replaceable class="PARAMETER">start</replaceable> ]
    </synopsis>

    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY, FOR UPDATE,
    or LIMIT clause.  (ORDER BY and LIMIT can be attached to a sub-expression
    if it is enclosed in parentheses.  Without parentheses, these clauses
    will be taken to apply to the result of the UNION, not to its right-hand
    input expression.)
   </para>
    
   <para>
    The UNION operator computes the collection (set union) of the rows
    returned by the queries involved.
    The two SELECTs that represent the direct operands of the UNION must
    produce the same number of columns, and corresponding columns must be
    of compatible data types.
   </para>
    
   <para>
    The result of UNION does not contain any duplicate rows
    unless the ALL option is specified.  ALL prevents elimination of
    duplicates.
   </para>
    
   <para>
    Multiple UNION operators in the same SELECT statement are
    evaluated left to right, unless otherwise indicated by parentheses.
   </para>
    
   <para>
    Currently, FOR UPDATE may not be specified either for a UNION result
    or for the inputs of a UNION.
   </para>

  </refsect2>

  <refsect2 id="SQL-INTERSECT">
   <refsect2info>
    <date>2000-12-11</date>
   </refsect2info>
   <title id="sql-intersect-title">
    INTERSECT Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> INTERSECT [ ALL ] <replaceable class="PARAMETER">table_query</replaceable>
    [ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
    [ LIMIT [ <replaceable class="PARAMETER">start</replaceable> , ] { <replaceable class="PARAMETER">count</replaceable> | ALL } ]
    [ OFFSET <replaceable class="PARAMETER">start</replaceable> ]
    </synopsis>
    
    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY, FOR UPDATE,
    or LIMIT clause.
   </para>

   <para>
    INTERSECT is similar to UNION, except that it produces only rows that
    appear in both query outputs, rather than rows that appear in either.
   </para>
    
   <para>
    The result of INTERSECT does not contain any duplicate rows
    unless the ALL option is specified.  With ALL, a row that has
    m duplicates in L and n duplicates in R will appear min(m,n) times.
   </para>
    
   <para>
    Multiple INTERSECT operators in the same SELECT statement are
    evaluated left to right, unless parentheses dictate otherwise.
    INTERSECT binds more tightly than UNION --- that is,
    A UNION B INTERSECT C will be read as 
    A UNION (B INTERSECT C) unless otherwise specified by parentheses.
   </para>
  </refsect2>

  <refsect2 id="SQL-EXCEPT">
   <refsect2info>
    <date>2000-12-11</date>
   </refsect2info>
   <title id="sql-except-title">
    EXCEPT Clause
   </title>
   <para>
    <synopsis>
<replaceable class="PARAMETER">table_query</replaceable> EXCEPT [ ALL ] <replaceable class="PARAMETER">table_query</replaceable>
    [ ORDER BY <replaceable class="PARAMETER">expression</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
    [ LIMIT [ <replaceable class="PARAMETER">start</replaceable> , ] { <replaceable class="PARAMETER">count</replaceable> | ALL } ]
    [ OFFSET <replaceable class="PARAMETER">start</replaceable> ]
    </synopsis>
    
    where
    <replaceable class="PARAMETER">table_query</replaceable>
    specifies any select expression without an ORDER BY, FOR UPDATE,
    or LIMIT clause.
   </para>

   <para>
    EXCEPT is similar to UNION, except that it produces only rows that
    appear in the left query's output but not in the right query's output.
   </para>
    
   <para>
    The result of EXCEPT does not contain any duplicate rows
    unless the ALL option is specified.  With ALL, a row that has
    m duplicates in L and n duplicates in R will appear max(m-n,0) times.
   </para>
    
   <para>
    Multiple EXCEPT operators in the same SELECT statement are
    evaluated left to right, unless parentheses dictate otherwise.
    EXCEPT binds at the same level as UNION.
   </para>
  </refsect2>

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

   <para>
<productname>Postgres</productname> allows one to omit 
the <command>FROM</command> clause from a query. This feature
was retained from the original PostQuel query language.  It has
a straightforward use to compute the results of simple constant
expressions:

  <programlisting>
SELECT 2+2;

 ?column?
----------
        4
  </programlisting>

Some other DBMSes cannot do this except by introducing a dummy one-row
table to do the select from.  A less obvious use is to abbreviate a
normal select from one or more tables:

  <programlisting>
SELECT distributors.* WHERE distributors.name = 'Westward';

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

This works because an implicit FROM item is added for each table that is
referenced in the query but not mentioned in FROM.  While this is a convenient
shorthand, it's easy to misuse.  For example, the query
  <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>Postgres</Productname> 7.1
and later will warn if the implicit-FROM feature is used in a query that also
contains an explicit FROM clause.

   </para>
  </refsect2>

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

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

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

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