class="parameter">sub-SELECT</replaceable> )
           } [, ...]

<phrase>and <replaceable class="parameter">merge_delete</replaceable> is:</phrase>

DELETE
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>MERGE</command> performs actions that modify rows in the
   target table identified as <replaceable class="parameter">target_table_name</replaceable>,
   using the <replaceable class="parameter">data_source</replaceable>.
   <command>MERGE</command> provides a single <acronym>SQL</acronym>
   statement that can conditionally <command>INSERT</command>,
   <command>UPDATE</command> or <command>DELETE</command> rows, a task
   that would otherwise require multiple procedural language statements.
  </para>

  <para>
   First, the <command>MERGE</command> command performs a join
   from <replaceable class="parameter">data_source</replaceable> to
   the target table
   producing zero or more candidate change rows.  For each candidate change
   row, the status of <literal>MATCHED</literal>,
   <literal>NOT MATCHED BY SOURCE</literal>,
   or <literal>NOT MATCHED [BY TARGET]</literal>
   is set just once, after which <literal>WHEN</literal> clauses are evaluated
   in the order specified.  For each candidate change row, the first clause to
   evaluate as true is executed.  No more than one <literal>WHEN</literal>
   clause is executed for any candidate change row.
  </para>

  <para>
   <command>MERGE</command> actions have the same effect as
   regular <command>UPDATE</command>, <command>INSERT</command>, or
   <command>DELETE</command> commands of the same names. The syntax of
   those commands is different, notably that there is no <literal>WHERE</literal>
   clause and no table name is specified.  All actions refer to the
   target table,
   though modifications to other tables may be made using triggers.
  </para>

  <para>
   When <literal>DO NOTHING</literal> is specified, the source row is
   skipped. Since actions are evaluated in their specified order, <literal>DO
   NOTHING</literal> can be handy to skip non-interesting source rows before
   more fine-grained handling.
  </para>

  <para>
   The optional <literal>RETURNING</literal> clause causes <command>MERGE</command>
   to compute and return value(s) based on each row inserted, updated, or
   deleted.  Any expression using the source or target table's columns, or
   the <link linkend="merge-action"><function>merge_action()</function></link>
   function can be computed.  When an <command>INSERT</command> or
   <command>UPDATE</command> action is performed, the new values of the target
   table's columns are used.  When a <command>DELETE</command> is performed,
   the old values of the target table's columns are used.  The syntax of the
   <literal>RETURNING</literal> list is identical to that of the output list
   of <command>SELECT</command>.
  </para>

  <para>
   There is no separate <literal>MERGE</literal> privilege.
   If you specify an update action, you must have the
   <literal>UPDATE</literal> privilege on the column(s)
   of the target table
   that are referred to in the <literal>SET</literal> clause.
   If you specify an insert action, you must have the <literal>INSERT</literal>
   privilege on the target table.
   If you specify a delete action, you must have the <literal>DELETE</literal>
   privilege on the target table.
   If you specify a <literal>DO NOTHING</literal> action, you must have
   the <literal>SELECT</literal> privilege on at least one column
   of the target table.
   You will also need <literal>SELECT</literal> privilege on any column(s)
   of the <replaceable class="parameter">data_source</replaceable> and
   of the target table referred to
   in any <literal>condition</literal> (including <literal>join_condition</literal>)
   or <literal>expression</literal>.
   Privileges are tested once at statement start and are checked
   whether or not particular <literal>WHEN</literal> clauses are executed.