any remaining columns from <replaceable>T2</replaceable>.
       </para>

       <para>
        <indexterm>
         <primary>join</primary>
         <secondary>natural</secondary>
        </indexterm>
        <indexterm>
         <primary>natural join</primary>
        </indexterm>
        Finally, <literal>NATURAL</literal> is a shorthand form of
        <literal>USING</literal>: it forms a <literal>USING</literal> list
        consisting of all column names that appear in both
        input tables.  As with <literal>USING</literal>, these columns appear
        only once in the output table.  If there are no common
        column names, <literal>NATURAL JOIN</literal> behaves like
        <literal>CROSS JOIN</literal>.
       </para>

       <note>
        <para>
         <literal>USING</literal> is reasonably safe from column changes
         in the joined relations since only the listed columns
         are combined.  <literal>NATURAL</literal> is considerably more risky since
         any schema changes to either relation that cause a new matching
         column name to be present will cause the join to combine that new
         column as well.
        </para>
       </note>
      </listitem>
     </varlistentry>
    </variablelist>

    <para>
     To put this together, assume we have tables <literal>t1</literal>:
<programlisting>
 num | name
-----+------
   1 | a
   2 | b
   3 | c
</programlisting>
     and <literal>t2</literal>:
<programlisting>
 num | value
-----+-------
   1 | xxx
   3 | yyy
   5 | zzz
</programlisting>
     then we get the following results for the various joins:
<screen>
<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 CROSS JOIN t2;</userinput>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   1 | a    |   3 | yyy
   1 | a    |   5 | zzz
   2 | b    |   1 | xxx
   2 | b    |   3 | yyy
   2 | b    |   5 | zzz
   3 | c    |   1 | xxx
   3 | c    |   3 | yyy
   3 | c    |   5 | zzz
(9 rows)

<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 INNER JOIN t2 ON t1.num = t2.num;</userinput>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   3 | c    |   3 | yyy
(2 rows)

<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 INNER JOIN t2 USING (num);</userinput>
 num | name | value
-----+------+-------
   1 | a    | xxx
   3 | c    | yyy
(2 rows)

<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 NATURAL INNER JOIN t2;</userinput>
 num | name | value
-----+------+-------
   1 | a    | xxx
   3 | c    | yyy
(2 rows)

<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num;</userinput>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   2 | b    |     |
   3 | c    |   3 | yyy
(3 rows)

<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 LEFT JOIN t2 USING (num);</userinput>
 num | name | value
-----+------+-------
   1 | a    | xxx
   2 | b    |
   3 | c    | yyy
(3 rows)

<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 RIGHT JOIN t2 ON t1.num = t2.num;</userinput>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   3 | c    |   3 | yyy
     |      |   5 | zzz
(3 rows)

<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 FULL JOIN t2 ON t1.num = t2.num;</userinput>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   2 | b    |     |
   3 | c    |   3 | yyy
     |      |   5 | zzz
(4 rows)
</screen>
    </para>

    <para>
     The join condition specified with <literal>ON</literal> can also contain
     conditions that do not relate directly to the join.  This can
     prove useful for some queries but needs to be thought out
     carefully.  For example:
<screen>
<prompt>=&gt;</prompt> <userinput>SELECT * FROM t1 LEFT JOIN t2 ON t1.num = t2.num AND t2.value = 'xxx';</userinput>
 num | name | num | value
-----+------+-----+-------
   1 | a    |   1 | xxx
   2 | b    |     |
   3 | c    |     |
(3 rows)
</screen>
     Notice that placing