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-$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.71 2003/11/01 01:56:29 petere Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/select.sgml,v 1.72 2003/11/04 00:34:45 tgl Exp $
PostgreSQL documentation
-->
<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> [, ...] ) ]
</synopsis>
-<comment>FIXME: This last syntax is incorrect if the join type is an
-INNER or OUTER join (in which case one of NATURAL, ON ..., or USING
-... is mandatory, not optional). What's the best way to fix
-this?</comment>
-
</refsynopsisdiv>
<refsect1>
<listitem>
<para>
- The actual output rows are computed the
+ 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">
</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>
<literal>DISTINCT</literal> eliminates duplicate rows from the
</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>
The <literal>FOR UPDATE</literal> clause causes the
</para>
<para>
- A <literal>JOIN</literal> clause, combines two
- <literal>FROM</> items. (Use parentheses if necessary to
- determine the order of nesting.)
+ 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 as you get from
- listing the two items at the top level of <literal>FROM</>.
+ 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.
+ (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</>.
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</>
- clauses own condition is considered while deciding which rows
+ clause's own condition is considered while deciding which rows
have matches. Outer conditions are applied afterwards.
</para>
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), or it can be an arbitrary
+ (<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>
Currently, <literal>FOR UPDATE</> may not be specified either for
- a <literal>UNION</> result or for the inputs of <literal>UNION</>.
+ a <literal>UNION</> result or for any input of a <literal>UNION</>.
</para>
</refsect2>
</synopsis>
<replaceable class="parameter">expression</replaceable> can be the
name or ordinal number of an output column
- (<command>SELECT</command> list), or it can be an arbitrary
+ (<command>SELECT</command> list item), or it can be an arbitrary
expression formed from input-column values.
</para>
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
- random order.
+ an implementation-dependent order.
</para>
<para>
<para>
Optionally one may add the key word <literal>ASC</> (ascending) or
- <literal>DESC</> (descending) after each expression in the
+ <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.
<para>
The <literal>LIMIT</literal> clause consists of two independent
- clauses:
+ 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, and <replaceable
+ maximum number of rows to return, while <replaceable
class="parameter">start</replaceable> specifies the number of rows
- to skip before starting to return 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>
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-FOR-UPDATE">
</para>
<para>
- This example shows how to obtain the union of the tables
+ 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 letter W in each table. Only distinct rows are wanted, so the
+ with the letter W in each table. Only distinct rows are wanted, so the
key word <literal>ALL</literal> is omitted.
<programlisting>
<para>
This example shows how to use a function in the <literal>FROM</>
- clause, both with and without a column definition list.
+ clause, both with and without a column definition list:
<programlisting>
CREATE FUNCTION distributors(int) RETURNS SETOF distributors AS '
</para>
<para>
- SQL99 uses a slightly different definition which is not upward compatible
+ SQL99 uses a slightly different definition which is not entirely upward
+ compatible
with SQL92. In most cases, however, <productname>PostgreSQL</productname>
will interpret an <literal>ORDER BY</literal> or <literal>GROUP
BY</literal> expression the same way SQL99 does.
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