A <literal>SELECT</literal> sub-query that produces as many output columns
      as are listed in the parenthesized column list preceding it.  The
      sub-query must yield no more than one row when executed.  If it
      yields one row, its column values are assigned to the target columns;
      if it yields no rows, NULL values are assigned to the target columns.
      If used in a <literal>WHEN MATCHED</literal> clause, the sub-query can
      refer to values from the original row in the target table, and values
      from the <replaceable class="parameter">data_source</replaceable> row.
      If used in a <literal>WHEN NOT MATCHED BY SOURCE</literal> clause, the
      sub-query can only refer to values from the original row in the target
      table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">output_alias</replaceable></term>
    <listitem>
     <para>
      An optional substitute name for <literal>OLD</literal> or
      <literal>NEW</literal> rows in the <literal>RETURNING</literal> list.
     </para>
     <para>
      By default, old values from the target table can be returned by writing
      <literal>OLD.<replaceable class="parameter">column_name</replaceable></literal>
      or <literal>OLD.*</literal>, and new values can be returned by writing
      <literal>NEW.<replaceable class="parameter">column_name</replaceable></literal>
      or <literal>NEW.*</literal>.  When an alias is provided, these names are
      hidden and the old or new rows must be referred to using the alias.
      For example <literal>RETURNING WITH (OLD AS o, NEW AS n) o.*, n.*</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">output_expression</replaceable></term>
    <listitem>
     <para>
      An expression to be computed and returned by the <command>MERGE</command>
      command after each row is changed (whether inserted, updated, or deleted).
      The expression can use any columns of the source or target tables, or the
      <link linkend="merge-action"><function>merge_action()</function></link>
      function to return additional information about the action executed.
     </para>
     <para>
      Writing <literal>*</literal> will return all columns from the source
      table, followed by all columns from the target table.  Often this will
      lead to a lot of duplication, since it is common for the source and
      target tables to have a lot of the same columns.  This can be avoided by
      qualifying the <literal>*</literal> with the name or alias of the source
      or target table.
     </para>
     <para>
      A column name or <literal>*</literal> may also be qualified using
      <literal>OLD</literal> or <literal>NEW</literal>, or the corresponding
      <replaceable class="parameter">output_alias</replaceable> for
      <literal>OLD</literal> or <literal>NEW</literal>, to cause old or new
      values from the target table to be returned.  An unqualified column
      name from the target table, or a column name or <literal>*</literal>
      qualified using the target table name or alias will return new values
      for <literal>INSERT</literal> and <literal>UPDATE</literal> actions, and
      old values for <literal>DELETE</literal> actions.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">output_name</replaceable></term>
    <listitem>
     <para>
      A name to use for a returned column.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>
 </refsect1>

 <refsect1>
  <title>Outputs</title>

  <para>
   On successful completion, a <command>MERGE</command> command returns a command
   tag of the form
<screen>
MERGE <replaceable class="parameter">total_count</replaceable>
</screen>
   The <replaceable class="parameter">total_count</replaceable> is the total
   number of rows changed (whether inserted, updated, or