so far as the
     current query is concerned — it is not allowed to refer to the
     table by the original name elsewhere in the query.  Thus, this is not
     valid:
<programlisting>
SELECT * FROM my_table AS m WHERE my_table.a &gt; 5;    -- wrong
</programlisting>
    </para>

    <para>
     Table aliases are mainly for notational convenience, but it is
     necessary to use them when joining a table to itself, e.g.:
<programlisting>
SELECT * FROM people AS mother JOIN people AS child ON mother.id = child.mother_id;
</programlisting>
    </para>

    <para>
     Parentheses are used to resolve ambiguities.  In the following example,
     the first statement assigns the alias <literal>b</literal> to the second
     instance of <literal>my_table</literal>, but the second statement assigns the
     alias to the result of the join:
<programlisting>
SELECT * FROM my_table AS a CROSS JOIN my_table AS b ...
SELECT * FROM (my_table AS a CROSS JOIN my_table) AS b ...
</programlisting>
    </para>

    <para>
     Another form of table aliasing gives temporary names to the columns of
     the table, as well as the table itself:
<synopsis>
FROM <replaceable>table_reference</replaceable> <optional>AS</optional> <replaceable>alias</replaceable> ( <replaceable>column1</replaceable> <optional>, <replaceable>column2</replaceable> <optional>, ...</optional></optional> )
</synopsis>
     If fewer column aliases are specified than the actual table has
     columns, the remaining columns are not renamed.  This syntax is
     especially useful for self-joins or subqueries.
    </para>

    <para>
     When an alias is applied to the output of a <literal>JOIN</literal>
     clause, the alias hides the original
     name(s) within the <literal>JOIN</literal>.  For example:
<programlisting>
SELECT a.* FROM my_table AS a JOIN your_table AS b ON ...
</programlisting>
     is valid SQL, but:
<programlisting>
SELECT a.* FROM (my_table AS a JOIN your_table AS b ON ...) AS c
</programlisting>
     is not valid; the table alias <literal>a</literal> is not visible
     outside the alias <literal>c</literal>.
    </para>
   </sect3>

   <sect3 id="queries-subqueries">
    <title>Subqueries</title>

    <indexterm zone="queries-subqueries">
     <primary>subquery</primary>
    </indexterm>

    <para>
     Subqueries specifying a derived table must be enclosed in
     parentheses.  They may be assigned a table alias name, and optionally
     column alias names (as in <xref linkend="queries-table-aliases"/>).
     For example:
<programlisting>
FROM (SELECT * FROM table1) AS alias_name
</programlisting>
    </para>

    <para>
     This example is equivalent to <literal>FROM table1 AS
     alias_name</literal>.  More interesting cases, which cannot be
     reduced to a plain join, arise when the subquery involves
     grouping or aggregation.
    </para>

    <para>
     A subquery can also be a <command>VALUES</command> list:
<programlisting>
FROM (VALUES ('anne', 'smith'), ('bob', 'jones'), ('joe', 'blow'))
     AS names(first, last)
</programlisting>
     Again, a table alias is optional.  Assigning alias names to the columns
     of the <command>VALUES</command> list is optional, but is good practice.
     For more information see <xref linkend="queries-values"/>.
    </para>

    <para>
     According to the SQL standard, a table alias name must be supplied
     for a subquery.  <productname>PostgreSQL</productname>
     allows <literal>AS</literal> and the alias to be omitted, but
     writing one is good practice in SQL code that might be ported to
     another system.
    </para>
   </sect3>

   <sect3 id="queries-tablefunctions">
    <title>Table Functions</title>

    <indexterm zone="queries-tablefunctions"><primary>table function</primary></indexterm>

    <indexterm zone="queries-tablefunctions">
     <primary>function</primary>
     <secondary>in the FROM clause</secondary>
    </indexterm>

    <para>
     Table functions are functions