Update SELECT documentation to have a more thorough discussion
of LIMIT, and fix a few other glitches too.
This commit is contained in:
Tom Lane
parent
be05edd812
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@ -1,5 +1,5 @@
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.24 2000/01/27 18:11:25 tgl Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.25 2000/02/21 01:13:52 tgl Exp $
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Postgres documentation
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-->
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@ -30,9 +30,9 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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[ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
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[ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
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[ { UNION [ ALL ] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable> ]
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[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
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[ FOR UPDATE [ OF class_name... ] ]
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[ LIMIT { count | ALL } [ { OFFSET | , } count ] ]
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[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
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[ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
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LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
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</synopsis>
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<refsect2 id="R2-SQL-SELECT-1">
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@ -59,10 +59,10 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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<listitem>
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<para>
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Specifies another name for a column or an expression using
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the AS clause. <replaceable class="PARAMETER">name</replaceable>
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cannot be used in the WHERE
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condition. It can, however, be referenced in associated
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ORDER BY or GROUP BY clauses.
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the AS clause. This name is primarily used to label the output
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column. The <replaceable class="PARAMETER">name</replaceable>
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cannot be used in the WHERE, GROUP BY, or HAVING clauses.
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It can, however, be referenced in ORDER BY clauses.
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</para>
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</listitem>
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</varlistentry>
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@ -245,18 +245,19 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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</para>
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<para>
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The UNION clause allows the result to be the collection of rows
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The UNION operator allows the result to be the collection of rows
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returned by the queries involved.
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(See <xref linkend="sql-union" endterm="sql-union-title">.)
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</para>
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<para>
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The INTERSECT give you the rows that are common to both queries.
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The INTERSECT operator gives you the rows that are common to both queries.
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(See <xref linkend="sql-intersect" endterm="sql-intersect-title">.)
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</para>
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<para>
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The EXCEPT give you the rows in the upper query not in the lower query.
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The EXCEPT operator gives you the rows returned by the first query but
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not the second query.
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(See <xref linkend="sql-except" endterm="sql-except-title">.)
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</para>
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@ -266,8 +267,9 @@ SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replac
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</para>
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<para>
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The LIMIT clause allows control over which rows are
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returned by the query.
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The LIMIT clause allows a subset of the rows produced by the query
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to be returned to the user.
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(See <xref linkend="sql-limit" endterm="sql-limit-title">.)
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</para>
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<para>
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@ -395,15 +397,15 @@ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, .
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of the column. This feature makes it possible to define an ordering
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on the basis of a column that does not have a proper name.
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This is never absolutely necessary because it is always possible
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assign a name
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to a calculated column using the AS clause, e.g.:
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to assign a name to a calculated column using the AS clause, e.g.:
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<programlisting>
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SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
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</programlisting></para>
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<para>
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From release 6.4 of PostgreSQL, the columns in the ORDER BY clause
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do not need to appear in the SELECT clause.
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From release 6.4 of PostgreSQL, it is also possible to ORDER BY
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arbitrary expressions, including fields that do not appear in the
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SELECT result list.
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Thus the following statement is now legal:
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<programlisting>
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SELECT name FROM distributors ORDER BY code;
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@ -413,7 +415,9 @@ SELECT name FROM distributors ORDER BY code;
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<para>
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Optionally one may add the keyword DESC (descending)
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or ASC (ascending) after each column name in the ORDER BY clause.
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If not specified, ASC is assumed by default.
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If not specified, ASC is assumed by default. Alternatively, a
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specific ordering operator name may be specified. ASC is equivalent
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to USING '<' and DESC is equivalent to USING '>'.
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</para>
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</refsect2>
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@ -436,10 +440,10 @@ SELECT name FROM distributors ORDER BY code;
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</para>
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<para>
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The UNION clause allows the result to be the collection of rows
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returned by the queries involved. (See UNION clause).
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The two tables that represent the direct operands of the UNION must
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have the same number of columns, and corresponding columns must be
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The UNION operator allows the result to be the collection of rows
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returned by the queries involved.
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The two SELECTs that represent the direct operands of the UNION must
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produce the same number of columns, and corresponding columns must be
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of compatible data types.
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</para>
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@ -476,16 +480,15 @@ SELECT name FROM distributors ORDER BY code;
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</para>
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<para>
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The INTERSECT clause allows the result to be all rows that are
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common to the involved queries.
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The two tables that represent the direct operands of the INTERSECT must
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have the same number of columns, and corresponding columns must be
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The INTERSECT operator gives you the rows that are common to both queries.
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The two SELECTs that represent the direct operands of the INTERSECT must
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produce the same number of columns, and corresponding columns must be
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of compatible data types.
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</para>
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<para>
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Multiple INTERSECT operators in the same SELECT statement are
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evaluated left to right.
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evaluated left to right, unless parentheses dictate otherwise.
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</para>
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</refsect2>
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@ -508,16 +511,65 @@ SELECT name FROM distributors ORDER BY code;
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</para>
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<para>
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The EXCEPT clause allows the result to be rows from the upper query
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that are not in the lower query. (See EXCEPT clause).
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The two tables that represent the direct operands of the EXCEPT must
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have the same number of columns, and corresponding columns must be
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The EXCEPT operator gives you the rows returned by the first query but
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not the second query.
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The two SELECTs that represent the direct operands of the EXCEPT must
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produce the same number of columns, and corresponding columns must be
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of compatible data types.
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</para>
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<para>
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Multiple EXCEPT operators in the same SELECT statement are
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evaluated left to right.
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evaluated left to right, unless parentheses dictate otherwise.
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</para>
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</refsect2>
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<refsect2 id="SQL-LIMIT">
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<refsect2info>
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<date>2000-02-20</date>
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</refsect2info>
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<title id="sql-limit-title">
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LIMIT Clause
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</title>
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<para>
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<synopsis>
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LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
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OFFSET <replaceable class="PARAMETER">start</replaceable>
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</synopsis>
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where
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<replaceable class="PARAMETER">count</replaceable> specifies the
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maximum number of rows to return, and
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<replaceable class="PARAMETER">start</replaceable> specifies the
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number of rows to skip before starting to return rows.
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</para>
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<para>
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LIMIT allows you to retrieve just a portion of the rows that are generated
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by the rest of the query. If a limit count is given, no more than that
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many rows will be returned. If an offset is given, that many rows will
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be skipped before starting to return rows.
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</para>
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<para>
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When using LIMIT, it is a good idea to use an ORDER BY clause that
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constrains the result rows into a unique order. Otherwise you will get
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an unpredictable subset of the query's rows --- you may be asking for
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the tenth through twentieth rows, but tenth through twentieth in what
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ordering? You don't know what ordering, unless you specified ORDER BY.
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</para>
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<para>
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As of Postgres 7.0, the
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query optimizer takes LIMIT into account when generating a query plan,
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so you are very likely to get different plans (yielding different row
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orders) depending on what you give for LIMIT and OFFSET. Thus, using
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different LIMIT/OFFSET values to select different subsets of a query
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result <emphasis>will give inconsistent results</emphasis> unless
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you enforce a predictable result ordering with ORDER BY. This is not
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a bug; it is an inherent consequence of the fact that SQL does not
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promise to deliver the results of a query in any particular order
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unless ORDER BY is used to constrain the order.
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</para>
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</refsect2>
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</refsect1>
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@ -624,7 +676,7 @@ SELECT * FROM distributors ORDER BY 2;
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This example shows how to obtain the union of the tables
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<literal>distributors</literal> and
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<literal>actors</literal>, restricting the results to those that begin
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with letter W in each table. Only distinct rows are to be used, so the
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with letter W in each table. Only distinct rows are wanted, so the
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ALL keyword is omitted:
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<programlisting>
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@ -709,16 +761,9 @@ SELECT distributors.* WHERE name = 'Westwood';
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parsing ambiguities
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in this context.</para>
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<para>
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In the <acronym>SQL92</acronym> standard, the new column name
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specified in an
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"AS" clause may be referenced in GROUP BY and HAVING clauses.
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This is not currently
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allowed in <productname>Postgres</productname>.
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</para>
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<para>
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The DISTINCT ON phrase is not part of <acronym>SQL92</acronym>.
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Nor are LIMIT and OFFSET.
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</para>
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</refsect3>
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@ -1,5 +1,5 @@
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select_into.sgml,v 1.4 1999/07/22 15:09:14 thomas Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/ref/select_into.sgml,v 1.5 2000/02/21 01:13:52 tgl Exp $
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Postgres documentation
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-->
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@ -22,16 +22,17 @@ Postgres documentation
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<date>1999-07-20</date>
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</refsynopsisdivinfo>
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<synopsis>
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SELECT [ ALL | DISTINCT ] <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
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INTO [TEMP] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
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[ FROM <replaceable class="PARAMETER">table</replaceable> [<replaceable class="PARAMETER">alias</replaceable>] [, ...] ]
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SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ]
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<replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
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[ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
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[ FROM <replaceable class="PARAMETER">table</replaceable> [ <replaceable class="PARAMETER">alias</replaceable> ] [, ...] ]
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[ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
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[ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
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[ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
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[ { UNION [ALL] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable>]
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[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
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[ FOR UPDATE [OF class_name...]]
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[ LIMIT count [OFFSET|, count]]
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[ { UNION [ ALL ] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable> ]
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[ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
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[ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
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LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
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</synopsis>
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<refsect2 id="R2-SQL-SELECTINTO-1">
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