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Rewrite discussion of ORDER BY to emphasize the SQL99 expression case

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instead of the SQL92 output-column-ID case.
This commit is contained in:
Tom Lane 2006-10-24 02:24:27 +00:00
parent 66f5264a2e
commit cfb3a42831
1 changed files with 39 additions and 36 deletions
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75
doc/src/sgml/queries.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.38 2006/10/23 18:10:31 petere Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.39 2006/10/24 02:24:27 tgl Exp $ -->
<chapter id="queries">
<title>Queries</title>
@ -514,8 +514,8 @@ SELECT * FROM my_table AS m WHERE my_table.a &gt; 5;
is not valid according to the SQL standard. In
<productname>PostgreSQL</productname> this will draw an error if the
<xref linkend="guc-add-missing-from"> configuration variable is
<literal>off</>. If it is <literal>on</>, an implicit table reference
will be added to the
<literal>off</> (as it is by default). If it is <literal>on</>,
an implicit table reference will be added to the
<literal>FROM</literal> clause, so the query is processed as if
it were written as
<programlisting>
@ -1224,38 +1224,17 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
<synopsis>
SELECT <replaceable>select_list</replaceable>
FROM <replaceable>table_expression</replaceable>
ORDER BY <replaceable>column1</replaceable> <optional>ASC | DESC</optional> <optional>, <replaceable>column2</replaceable> <optional>ASC | DESC</optional> ...</optional>
ORDER BY <replaceable>sort_expression1</replaceable> <optional>ASC | DESC</optional> <optional>, <replaceable>sort_expression2</replaceable> <optional>ASC | DESC</optional> ...</optional>
</synopsis>
<replaceable>column1</replaceable>, etc., refer to select list
columns. These can be either the output name of a column (see
<xref linkend="queries-column-labels">) or the number of a column. Some
examples:
The sort expression(s) can be any expression that would be valid in the
query's select list. An example is
<programlisting>
SELECT a, b FROM table1 ORDER BY a;
SELECT a + b AS sum, c FROM table1 ORDER BY sum;
SELECT a, sum(b) FROM table1 GROUP BY a ORDER BY 1;
SELECT a, b FROM table1 ORDER BY a + b, c;
</programlisting>
</para>
<para>
As an extension to the SQL standard, <productname>PostgreSQL</productname> also allows ordering
by arbitrary expressions:
<programlisting>
SELECT a, b FROM table1 ORDER BY a + b;
</programlisting>
References to column names of the <literal>FROM</> clause that are
not present in the select list are also allowed:
<programlisting>
SELECT a FROM table1 ORDER BY b;
</programlisting>
But these extensions do not work in queries involving
<literal>UNION</>, <literal>INTERSECT</>, or <literal>EXCEPT</>,
and are not portable to other SQL databases.
</para>
<para>
Each column specification may be followed by an optional
<literal>ASC</> or <literal>DESC</> to set the sort direction to
When more than one expression is specified,
the later values are used to sort rows that are equal according to the
earlier values. Each expression may be followed by an optional
<literal>ASC</> or <literal>DESC</> keyword to set the sort direction to
ascending or descending. <literal>ASC</> order is the default.
Ascending order puts smaller values first, where
<quote>smaller</quote> is defined in terms of the
@ -1264,7 +1243,7 @@ SELECT a FROM table1 ORDER BY b;
<footnote>
<para>
Actually, <productname>PostgreSQL</> uses the <firstterm>default B-tree
operator class</> for the column's data type to determine the sort
operator class</> for the expression's data type to determine the sort
ordering for <literal>ASC</> and <literal>DESC</>. Conventionally,
data types will be set up so that the <literal>&lt;</literal> and
<literal>&gt;</literal> operators correspond to this sort ordering,
@ -1275,9 +1254,32 @@ SELECT a FROM table1 ORDER BY b;
</para>
<para>
If more than one sort column is specified, the later entries are
used to sort rows that are equal under the order imposed by the
earlier sort columns.
For backwards compatibility with the SQL92 version of the standard,
a <replaceable>sort_expression</> can instead be the name or number
of an output column, as in
<programlisting>
SELECT a + b AS sum, c FROM table1 ORDER BY sum;
SELECT a, max(b) FROM table1 GROUP BY a ORDER BY 1;
</programlisting>
both of which sort by the first output column. Note that an output
column name has to stand alone, it's not allowed as part of an expression
&mdash; for example, this is <emphasis>not</> correct:
<programlisting>
SELECT a + b AS sum, c FROM table1 ORDER BY sum + c; -- wrong
</programlisting>
This restriction is made to reduce ambiguity. There is still
ambiguity if an <literal>ORDER BY</> item is a simple name that
could match either an output column name or a column from the table
expression. The output column is used in such cases. This would
only cause confusion if you use <literal>AS</> to rename an output
column to match some other table column's name.
</para>
<para>
<literal>ORDER BY</> can be applied to the result of a
<literal>UNION</>, <literal>INTERSECT</>, or <literal>EXCEPT</>
combination, but in this case it is only permitted to sort by
output column names or numbers, not by expressions.
</para>
</sect1>
@ -1299,6 +1301,7 @@ SELECT a FROM table1 ORDER BY b;
<synopsis>
SELECT <replaceable>select_list</replaceable>
FROM <replaceable>table_expression</replaceable>
<optional> ORDER BY <replaceable>sort_expression1</replaceable> <optional>ASC | DESC</optional> <optional>, <replaceable>sort_expression2</replaceable> <optional>ASC | DESC</optional> ...</optional> </optional>
<optional> LIMIT { <replaceable>number</replaceable> | ALL } </optional> <optional> OFFSET <replaceable>number</replaceable> </optional>
</synopsis>
</para>