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+$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.25 2000/02/21 01:13:52 tgl Exp $
Postgres documentation
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[ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
[ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
[ { UNION [ ALL ] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable> ]
- [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
- [ FOR UPDATE [ OF class_name... ] ]
- [ LIMIT { count | ALL } [ { OFFSET | , } count ] ]
+ [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
+ [ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
+ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
</synopsis>
<refsect2 id="R2-SQL-SELECT-1">
<listitem>
<para>
Specifies another name for a column or an expression using
- the AS clause. <replaceable class="PARAMETER">name</replaceable>
- cannot be used in the WHERE
- condition. It can, however, be referenced in associated
- ORDER BY or GROUP BY clauses.
+ the AS clause. This name is primarily used to label the output
+ column. The <replaceable class="PARAMETER">name</replaceable>
+ cannot be used in the WHERE, GROUP BY, or HAVING clauses.
+ It can, however, be referenced in ORDER BY clauses.
</para>
</listitem>
</varlistentry>
</para>
<para>
- The UNION clause allows the result to be the collection of rows
+ The UNION operator allows the result to be the collection of rows
returned by the queries involved.
(See <xref linkend="sql-union" endterm="sql-union-title">.)
</para>
<para>
- The INTERSECT give you the rows that are common to both queries.
+ The INTERSECT operator gives you the rows that are common to both queries.
(See <xref linkend="sql-intersect" endterm="sql-intersect-title">.)
</para>
<para>
- The EXCEPT give you the rows in the upper query not in the lower query.
+ The EXCEPT operator gives you the rows returned by the first query but
+ not the second query.
(See <xref linkend="sql-except" endterm="sql-except-title">.)
</para>
</para>
<para>
- The LIMIT clause allows control over which rows are
- returned by the query.
+ 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>
of the 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
- assign a name
- to a calculated column using the AS clause, e.g.:
+ to assign a name to a calculated column using the AS clause, e.g.:
<programlisting>
SELECT title, date_prod + 1 AS newlen FROM films ORDER BY newlen;
</programlisting></para>
<para>
- From release 6.4 of PostgreSQL, the columns in the ORDER BY clause
- do not need to appear in the SELECT clause.
+ From release 6.4 of PostgreSQL, it is also possible to ORDER BY
+ arbitrary expressions, including fields that do not appear in the
+ SELECT result list.
Thus the following statement is now legal:
<programlisting>
SELECT name FROM distributors ORDER BY code;
<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.
+ If not specified, ASC is assumed by default. Alternatively, a
+ specific ordering operator name may be specified. ASC is equivalent
+ to USING '<' and DESC is equivalent to USING '>'.
</para>
</refsect2>
</para>
<para>
- The UNION clause allows the result to be the collection of rows
- returned by the queries involved. (See UNION clause).
- The two tables that represent the direct operands of the UNION must
- have the same number of columns, and corresponding columns must be
+ The UNION operator allows the result to be the collection of 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>
<para>
- The INTERSECT clause allows the result to be all rows that are
- common to the involved queries.
- The two tables that represent the direct operands of the INTERSECT must
- have the same number of columns, and corresponding columns must be
+ The INTERSECT operator gives you the rows that are common to both queries.
+ The two SELECTs that represent the direct operands of the INTERSECT must
+ produce the same number of columns, and corresponding columns must be
of compatible data types.
</para>
<para>
Multiple INTERSECT operators in the same SELECT statement are
- evaluated left to right.
+ evaluated left to right, unless parentheses dictate otherwise.
</para>
</refsect2>
</para>
<para>
- The EXCEPT clause allows the result to be rows from the upper query
- that are not in the lower query. (See EXCEPT clause).
- The two tables that represent the direct operands of the EXCEPT must
- have the same number of columns, and corresponding columns must be
+ The EXCEPT operator gives you the rows returned by the first query but
+ not the second query.
+ The two SELECTs that represent the direct operands of the EXCEPT must
+ produce the same number of columns, and corresponding columns must be
of compatible data types.
</para>
<para>
Multiple EXCEPT operators in the same SELECT statement are
- evaluated left to right.
+ evaluated left to right, unless parentheses dictate otherwise.
+ </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">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
+ 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 specified ORDER BY.
+ </para>
+
+ <para>
+ As of Postgres 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 give 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>
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 to be used, so the
+ with letter W in each table. Only distinct rows are wanted, so the
ALL keyword is omitted:
<programlisting>
parsing ambiguities
in this context.</para>
- <para>
- In the <acronym>SQL92</acronym> standard, the new column name
- specified in an
- "AS" clause may be referenced in GROUP BY and HAVING clauses.
- This is not currently
- allowed in <productname>Postgres</productname>.
- </para>
-
<para>
The DISTINCT ON phrase is not part of <acronym>SQL92</acronym>.
+ Nor are LIMIT and OFFSET.
</para>
</refsect3>
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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 $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/select_into.sgml,v 1.5 2000/02/21 01:13:52 tgl Exp $
Postgres documentation
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<date>1999-07-20</date>
</refsynopsisdivinfo>
<synopsis>
-SELECT [ ALL | DISTINCT ] <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
- INTO [TEMP] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
- [ FROM <replaceable class="PARAMETER">table</replaceable> [<replaceable class="PARAMETER">alias</replaceable>] [, ...] ]
+SELECT [ ALL | DISTINCT [ ON ( <replaceable class="PARAMETER">expression</replaceable> [, ...] ) ] ]
+ <replaceable class="PARAMETER">expression</replaceable> [ AS <replaceable class="PARAMETER">name</replaceable> ] [, ...]
+ [ INTO [ TEMPORARY | TEMP ] [ TABLE ] <replaceable class="PARAMETER">new_table</replaceable> ]
+ [ FROM <replaceable class="PARAMETER">table</replaceable> [ <replaceable class="PARAMETER">alias</replaceable> ] [, ...] ]
[ WHERE <replaceable class="PARAMETER">condition</replaceable> ]
[ GROUP BY <replaceable class="PARAMETER">column</replaceable> [, ...] ]
[ HAVING <replaceable class="PARAMETER">condition</replaceable> [, ...] ]
- [ { UNION [ALL] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable>]
- [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC ] [, ...] ]
- [ FOR UPDATE [OF class_name...]]
- [ LIMIT count [OFFSET|, count]]
+ [ { UNION [ ALL ] | INTERSECT | EXCEPT } <replaceable class="PARAMETER">select</replaceable> ]
+ [ ORDER BY <replaceable class="PARAMETER">column</replaceable> [ ASC | DESC | USING <replaceable class="PARAMETER">operator</replaceable> ] [, ...] ]
+ [ FOR UPDATE [ OF <replaceable class="PARAMETER">class_name</replaceable> [, ...] ] ]
+ LIMIT { <replaceable class="PARAMETER">count</replaceable> | ALL } [ { OFFSET | , } <replaceable class="PARAMETER">start</replaceable> ]
</synopsis>
<refsect2 id="R2-SQL-SELECTINTO-1">
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