-<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.28 2003/11/29 19:51:37 pgsql Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/queries.sgml,v 1.29 2004/03/03 22:22:24 neilc Exp $ -->
<chapter id="queries">
<title>Queries</title>
<title>The <literal>FROM</literal> Clause</title>
<para>
- The <literal>FROM</> clause derives a table from one or more other
- tables given in a comma-separated table reference list.
+ The <xref linkend="sql-from" endterm="sql-from-title"> derives a
+ table from one or more other tables given in a comma-separated
+ table reference list.
<synopsis>
FROM <replaceable>table_reference</replaceable> <optional>, <replaceable>table_reference</replaceable> <optional>, ...</optional></optional>
</synopsis>
</indexterm>
<para>
- The syntax of the <literal>WHERE</> clause is
+ The syntax of the <xref linkend="sql-where"
+ endterm="sql-where-title"> clause is
<synopsis>
WHERE <replaceable>search_condition</replaceable>
</synopsis>
</synopsis>
<para>
- The <literal>GROUP BY</> clause is used to group together those rows in
- a table that share the same values in all the columns listed. The
- order in which the columns are listed does not matter. The
- purpose is to reduce each group of rows sharing common values into
- one group row that is representative of all rows in the group.
- This is done to eliminate redundancy in the output and/or compute
- aggregates that apply to these groups. For instance:
+ The <xref linkend="sql-groupby" endterm="sql-groupby-title"> is
+ used to group together those rows in a table that share the same
+ values in all the columns listed. The order in which the columns
+ are listed does not matter. The purpose is to reduce each group
+ of rows sharing common values into one group row that is
+ representative of all rows in the group. This is done to
+ eliminate redundancy in the output and/or compute aggregates that
+ apply to these groups. For instance:
<screen>
<prompt>=></> <userinput>SELECT * FROM test1;</>
x | y
<synopsis>
SELECT DISTINCT <replaceable>select_list</replaceable> ...
</synopsis>
- (Instead of <literal>DISTINCT</> the word <literal>ALL</literal>
+ (Instead of <literal>DISTINCT</> the key word <literal>ALL</literal>
can be used to select the default behavior of retaining all rows.)
</para>
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PostgreSQL documentation
-->
</para>
<para>
- <literal>FROM</literal>-clause elements can contain:
+ The <literal>FROM</literal> clause can contain the following
+ elements:
<variablelist>
<varlistentry>
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.
+ <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 result as you get from
listing the two items at the top level of <literal>FROM</>,
- but restricted by the join condition (if any).
+ 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.
These join types are just a notational convenience, since they
<literal>INTERSECT</>, or <literal>EXCEPT</> clause may only
specify an output column name or number, not an expression.
</para>
-
+
<para>
If an <literal>ORDER BY</> expression is a simple name that
matches both a result column name and an input column name,
<!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/update.sgml,v 1.27 2003/11/29 19:51:39 pgsql Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/update.sgml,v 1.28 2004/03/03 22:22:24 neilc Exp $
PostgreSQL documentation
-->
<para>
<command>UPDATE</command> changes the values of the specified
columns in all rows that satisfy the condition. Only the columns to
- be modified need be mentioned in the statement; columns not explicitly
- <literal>SET</> retain their previous values.
+ be modified need be mentioned in the <literal>SET</literal> clause;
+ columns not explicitly modified retain their previous values.
</para>
<para>
clause.
</para>
+ <para>
+ There are two ways to modify a table using information contained in
+ other tables in the database: using sub-selects, or specifying
+ additional tables in the <literal>FROM</literal> clause. Which
+ technique is more appropriate depends on the specific
+ circumstances.
+ </para>
+
<para>
You must have the <literal>UPDATE</literal> privilege on the table
to update it, as well as the <literal>SELECT</literal>
<listitem>
<para>
A list of table expressions, allowing columns from other tables
- to appear in the <literal>WHERE</> condition and the update expressions.
+ to appear in the <literal>WHERE</> condition and the update
+ expressions. This is similar to the list of tables that can be
+ specified in the <xref linkend="sql-from"
+ endterm="sql-from-title"> of a <command>SELECT</command>
+ statement; for example, an alias for the table name can be
+ specified.
</para>
</listitem>
</varlistentry>
<para>
Change the word <literal>Drama</> to <literal>Dramatic</> in the
- column <structfield>kind</> of the table <literal>films</literal>:
+ column <structfield>kind</> of the table <structname>films</structname>:
<programlisting>
UPDATE films SET kind = 'Dramatic' WHERE kind = 'Drama';
<para>
Adjust temperature entries and reset precipitation to its default
- value in one row of the table <literal>weather</literal>:
+ value in one row of the table <structname>weather</structname>:
<programlisting>
UPDATE weather SET temp_lo = temp_lo+1, temp_hi = temp_lo+15, prcp = DEFAULT
</programlisting>
</para>
+ <para>
+ Increment the sales count of the salesperson who manages the
+ account for Acme Corporation, using the <literal>FROM</literal>
+ clause syntax:
+<programlisting>
+UPDATE employees SET sales_count = sales_count + 1 FROM accounts
+ WHERE accounts.name = 'Acme Corporation'
+ AND employees.id = accounts.sales_person;
+</programlisting>
+
+ Perform the same operation, using a sub-select in the
+ <literal>WHERE</literal> clause:
+<programlisting>
+UPDATE employees SET sales_count = sales_count + 1 WHERE id =
+ (SELECT sales_person FROM accounts WHERE name = 'Acme Corporation');
+</programlisting>
+ </para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
- This command conforms to the SQL standard. The
+ This command conforms to the <acronym>SQL</acronym> standard. The
<literal>FROM</literal> clause is a
<productname>PostgreSQL</productname> extension.
</para>
<!--
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+$PostgreSQL: pgsql/doc/src/sgml/trigger.sgml,v 1.35 2004/03/03 22:22:24 neilc Exp $
-->
<chapter id="triggers">
<para>
The function <function>trigf</> reports the number of rows in the
- table <literal>ttest</> and skips the actual operation if the
+ table <structname>ttest</> and skips the actual operation if the
command attempts to insert a null value into the column
- <literal>x</>. (So the trigger acts as a not-null constraint but
+ <structfield>x</>. (So the trigger acts as a not-null constraint but
doesn't abort the transaction.)
</para>
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