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doc/src/sgml/ref/merge.sgml
PostgreSQL documentation
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<refentry id="sql-merge">
 <indexterm zone="sql-merge">
  <primary>MERGE</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>MERGE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>MERGE</refname>
  <refpurpose>conditionally insert, update, or delete rows of a table</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
[ WITH <replaceable class="parameter">with_query</replaceable> [, ...] ]
MERGE INTO [ ONLY ] <replaceable class="parameter">target_table_name</replaceable> [ * ] [ [ AS ] <replaceable class="parameter">target_alias</replaceable> ]
    USING <replaceable class="parameter">data_source</replaceable> ON <replaceable class="parameter">join_condition</replaceable>
    <replaceable class="parameter">when_clause</replaceable> [...]
    [ RETURNING [ WITH ( { OLD | NEW } AS <replaceable class="parameter">output_alias</replaceable> [, ...] ) ]
                { * | <replaceable class="parameter">output_expression</replaceable> [ [ AS ] <replaceable class="parameter">output_name</replaceable> ] } [, ...] ]

<phrase>where <replaceable class="parameter">data_source</replaceable> is:</phrase>

    { [ ONLY ] <replaceable class="parameter">source_table_name</replaceable> [ * ] | ( <replaceable class="parameter">source_query</replaceable> ) } [ [ AS ] <replaceable class="parameter">source_alias</replaceable> ]

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

    { WHEN MATCHED [ AND <replaceable class="parameter">condition</replaceable> ] THEN { <replaceable class="parameter">merge_update</replaceable> | <replaceable class="parameter">merge_delete</replaceable> | DO NOTHING } |
      WHEN NOT MATCHED BY SOURCE [ AND <replaceable class="parameter">condition</replaceable> ] THEN { <replaceable class="parameter">merge_update</replaceable> | <replaceable class="parameter">merge_delete</replaceable> | DO NOTHING } |
      WHEN NOT MATCHED [ BY TARGET ] [ AND <replaceable class="parameter">condition</replaceable> ] THEN { <replaceable class="parameter">merge_insert</replaceable> | DO NOTHING } }

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

    INSERT [( <replaceable class="parameter">column_name</replaceable> [, ...] )]
        [ OVERRIDING { SYSTEM | USER } VALUE ]
        { VALUES ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) | DEFAULT VALUES }

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

    UPDATE SET { <replaceable class="parameter">column_name</replaceable> = { <replaceable class="parameter">expression</replaceable> | DEFAULT } |
                 ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = [ ROW ] ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] ) |
                 ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = ( <replaceable 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.  By default, when an <command>INSERT</command> or
   <command>UPDATE</command> action is performed, the new values of the target
   table's columns are used, and when a <command>DELETE</command> is performed,
   the old values of the target table's columns are used, but it is also
   possible to explicitly request old and new values.  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.
  </para>

  <para>
   <command>MERGE</command> is not supported if the
   target table is a
   materialized view, foreign table, or if it has any
   rules defined on it.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">with_query</replaceable></term>
    <listitem>
     <para>
      The <literal>WITH</literal> clause allows you to specify one or more
      subqueries that can be referenced by name in the <command>MERGE</command>
      query. See <xref linkend="queries-with"/> and <xref linkend="sql-select"/>
      for details.  Note that <literal>WITH RECURSIVE</literal> is not supported
      by <command>MERGE</command>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">target_table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the target table or view to
      merge into.  If <literal>ONLY</literal> is specified before a table
      name, matching rows are updated or deleted in the named table only.  If
      <literal>ONLY</literal> is not specified, matching rows are also updated
      or deleted in any tables inheriting from the named table.  Optionally,
      <literal>*</literal> can be specified after the table name to explicitly
      indicate that descendant tables are included.  The
      <literal>ONLY</literal> keyword and <literal>*</literal> option do not
      affect insert actions, which always insert into the named table only.
     </para>

     <para>
      If <replaceable class="parameter">target_table_name</replaceable> is a
      view, it must either be automatically updatable with no
      <literal>INSTEAD OF</literal> triggers, or it must have
      <literal>INSTEAD OF</literal> triggers for every type of action
      (<literal>INSERT</literal>, <literal>UPDATE</literal>, and
      <literal>DELETE</literal>) specified in the <literal>WHEN</literal>
      clauses.  Views with rules are not supported.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">target_alias</replaceable></term>
    <listitem>
     <para>
      A substitute name for the target table. When an alias is
      provided, it completely hides the actual name of the table.  For
      example, given <literal>MERGE INTO foo AS f</literal>, the remainder of the
      <command>MERGE</command> statement must refer to this table as
      <literal>f</literal> not <literal>foo</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">source_table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the source table, view, or
      transition table.  If <literal>ONLY</literal> is specified before the
      table name, matching rows are included from the named table only.  If
      <literal>ONLY</literal> is not specified, matching rows are also included
      from any tables inheriting from the named table.  Optionally,
      <literal>*</literal> can be specified after the table name to explicitly
      indicate that descendant tables are included.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">source_query</replaceable></term>
    <listitem>
     <para>
      A query (<command>SELECT</command> statement or <command>VALUES</command>
      statement) that supplies the rows to be merged into the
      target table.
      Refer to the <xref linkend="sql-select"/>
      statement or <xref linkend="sql-values"/>
      statement for a description of the syntax.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">source_alias</replaceable></term>
    <listitem>
     <para>
      A substitute name for the data source. When an alias is
      provided, it completely hides the actual name of the table or the fact
      that a query was issued.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">join_condition</replaceable></term>
    <listitem>
     <para>
      <replaceable class="parameter">join_condition</replaceable> is
      an expression resulting in a value of type
      <type>boolean</type> (similar to a <literal>WHERE</literal>
      clause) that specifies which rows in the
      <replaceable class="parameter">data_source</replaceable>
      match rows in the target table.
     </para>
     <warning>
      <para>
       Only columns from the target table
       that attempt to match <replaceable class="parameter">data_source</replaceable>
       rows should appear in <replaceable class="parameter">join_condition</replaceable>.
       <replaceable class="parameter">join_condition</replaceable> subexpressions that
       only reference the target table's
       columns can affect which action is taken, often in surprising ways.
      </para>
      <para>
       If both <literal>WHEN NOT MATCHED BY SOURCE</literal> and
       <literal>WHEN NOT MATCHED [BY TARGET]</literal> clauses are specified,
       the <command>MERGE</command> command will perform a <literal>FULL</literal>
       join between <replaceable class="parameter">data_source</replaceable>
       and the target table.  For this to work, at least one
       <replaceable class="parameter">join_condition</replaceable> subexpression
       must use an operator that can support a hash join, or all of the
       subexpressions must use operators that can support a merge join.
      </para>
     </warning>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">when_clause</replaceable></term>
    <listitem>
     <para>
      At least one <literal>WHEN</literal> clause is required.
     </para>
     <para>
      The <literal>WHEN</literal> clause may specify <literal>WHEN MATCHED</literal>,
      <literal>WHEN NOT MATCHED BY SOURCE</literal>, or
      <literal>WHEN NOT MATCHED [BY TARGET]</literal>.
      Note that the <acronym>SQL</acronym> standard only defines
      <literal>WHEN MATCHED</literal> and <literal>WHEN NOT MATCHED</literal>
      (which is defined to mean no matching target row).
      <literal>WHEN NOT MATCHED BY SOURCE</literal> is an extension to the
      <acronym>SQL</acronym> standard, as is the option to append
      <literal>BY TARGET</literal> to <literal>WHEN NOT MATCHED</literal>, to
      make its meaning more explicit.
     </para>
     <para>
      If the <literal>WHEN</literal> clause specifies <literal>WHEN MATCHED</literal>
      and the candidate change row matches a row in the
      <replaceable class="parameter">data_source</replaceable> to a row in the
      target table, the <literal>WHEN</literal> clause is executed if the
      <replaceable class="parameter">condition</replaceable> is
      absent or it evaluates to <literal>true</literal>.
     </para>
     <para>
      If the <literal>WHEN</literal> clause specifies
      <literal>WHEN NOT MATCHED BY SOURCE</literal> and the candidate change
      row represents a row in the target table that does not match a row in the
      <replaceable class="parameter">data_source</replaceable>, the
      <literal>WHEN</literal> clause is executed if the
      <replaceable class="parameter">condition</replaceable> is
      absent or it evaluates to <literal>true</literal>.
     </para>
     <para>
      If the <literal>WHEN</literal> clause specifies
      <literal>WHEN NOT MATCHED [BY TARGET]</literal> and the candidate change
      row represents a row in the
      <replaceable class="parameter">data_source</replaceable> that does not
      match a row in the target table,
      the <literal>WHEN</literal> clause is executed if the
      <replaceable class="parameter">condition</replaceable> is
      absent or it evaluates to <literal>true</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">condition</replaceable></term>
    <listitem>
     <para>
      An expression that returns a value of type <type>boolean</type>.
      If this expression for a <literal>WHEN</literal> clause
      returns <literal>true</literal>, then the action for that clause
      is executed for that row.
     </para>
     <para>
      A condition on a <literal>WHEN MATCHED</literal> clause can refer to columns
      in both the source and the target relations. A condition on a
      <literal>WHEN NOT MATCHED BY SOURCE</literal> clause can only refer to
      columns from the target relation, since by definition there is no matching
      source row. A condition on a <literal>WHEN NOT MATCHED [BY TARGET]</literal>
      clause can only refer to columns from
      the source relation, since by definition there is no matching target row.
      Only the system attributes from the target table are accessible.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">merge_insert</replaceable></term>
    <listitem>
     <para>
      The specification of an <literal>INSERT</literal> action that inserts
      one row into the target table.
      The target column names can be listed in any order. If no list of
      column names is given at all, the default is all the columns of the
      table in their declared order.
     </para>
     <para>
      Each column not present in the explicit or implicit column list will be
      filled with a default value, either its declared default value
      or null if there is none.
     </para>
     <para>
      If the target table
      is a partitioned table, each row is routed to the appropriate partition
      and inserted into it.
      If the target table
      is a partition, an error will occur if any input row violates the
      partition constraint.
     </para>
     <para>
      Column names may not be specified more than once.
      <command>INSERT</command> actions cannot contain sub-selects.
     </para>
     <para>
      Only one <literal>VALUES</literal> clause can be specified.
      The <literal>VALUES</literal> clause can only refer to columns from
      the source relation, since by definition there is no matching target row.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">merge_update</replaceable></term>
    <listitem>
     <para>
      The specification of an <literal>UPDATE</literal> action that updates
      the current row of the target table.
      Column names may not be specified more than once.
     </para>
     <para>
      Neither a table name nor a <literal>WHERE</literal> clause are allowed.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">merge_delete</replaceable></term>
    <listitem>
     <para>
      Specifies a <literal>DELETE</literal> action that deletes the current row
      of the target table.
      Do not include the table name or any other clauses, as you would normally
      do with a <xref linkend="sql-delete"/> command.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">column_name</replaceable></term>
    <listitem>
     <para>
      The name of a column in the target table.  The column name
      can be qualified with a subfield name or array subscript, if
      needed.  (Inserting into only some fields of a composite
      column leaves the other fields null.)
      Do not include the table's name in the specification
      of a target column.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>OVERRIDING SYSTEM VALUE</literal></term>
    <listitem>
     <para>
      Without this clause, it is an error to specify an explicit value
      (other than <literal>DEFAULT</literal>) for an identity column defined
      as <literal>GENERATED ALWAYS</literal>.  This clause overrides that
      restriction.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>OVERRIDING USER VALUE</literal></term>
    <listitem>
     <para>
      If this clause is specified, then any values supplied for identity
      columns defined as <literal>GENERATED BY DEFAULT</literal> are ignored
      and the default sequence-generated values are applied.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEFAULT VALUES</literal></term>
    <listitem>
     <para>
      All columns will be filled with their default values.
      (An <literal>OVERRIDING</literal> clause is not permitted in this
      form.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">expression</replaceable></term>
    <listitem>
     <para>
      An expression to assign to the column.  If used in a
      <literal>WHEN MATCHED</literal> clause, the expression can use 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
      expression can only use values from the original row in the target table.
      If used in a <literal>WHEN NOT MATCHED [BY TARGET]</literal> clause, the
      expression can only use values from the
      <replaceable class="parameter">data_source</replaceable> row.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEFAULT</literal></term>
    <listitem>
     <para>
      Set the column to its default value (which will be <literal>NULL</literal>
      if no specific default expression has been assigned to it).
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">sub-SELECT</replaceable></term>
    <listitem>
     <para>
      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 deleted).
   If <replaceable class="parameter">total_count</replaceable> is 0, no rows
   were changed in any way.
  </para>

  <para>
   If the <command>MERGE</command> command contains a <literal>RETURNING</literal>
   clause, the result will be similar to that of a <command>SELECT</command>
   statement containing the columns and values defined in the
   <literal>RETURNING</literal> list, computed over the row(s) inserted, updated,
   or deleted by the command.
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   The following steps take place during the execution of
   <command>MERGE</command>.
    <orderedlist>
     <listitem>
      <para>
       Perform any <literal>BEFORE STATEMENT</literal> triggers for all
       actions specified, whether or not their <literal>WHEN</literal>
       clauses match.
      </para>
     </listitem>
     <listitem>
      <para>
       Perform a join from source to target table.
       The resulting query will be optimized normally and will produce
       a set of candidate change rows. For each candidate change row,
       <orderedlist>
        <listitem>
         <para>
          Evaluate whether each row is <literal>MATCHED</literal>,
          <literal>NOT MATCHED BY SOURCE</literal>, or
          <literal>NOT MATCHED [BY TARGET]</literal>.
         </para>
        </listitem>
        <listitem>
         <para>
          Test each <literal>WHEN</literal> condition in the order
          specified until one returns true.
         </para>
        </listitem>
        <listitem>
         <para>
          When a condition returns true, perform the following actions:
          <orderedlist>
           <listitem>
            <para>
             Perform any <literal>BEFORE ROW</literal> triggers that fire
             for the action's event type.
            </para>
           </listitem>
           <listitem>
            <para>
             Perform the specified action, invoking any check constraints on the
             target table.
            </para>
           </listitem>
           <listitem>
            <para>
             Perform any <literal>AFTER ROW</literal> triggers that fire for
             the action's event type.
            </para>
           </listitem>
          </orderedlist>
          If the target relation is a view with <literal>INSTEAD OF ROW</literal>
          triggers for the action's event type, they are used to perform the
          action instead.
         </para>
        </listitem>
       </orderedlist></para>
     </listitem>
     <listitem>
      <para>
       Perform any <literal>AFTER STATEMENT</literal> triggers for actions
       specified, whether or not they actually occur.  This is similar to the
       behavior of an <command>UPDATE</command> statement that modifies no rows.
      </para>
     </listitem>
    </orderedlist>
   In summary, statement triggers for an event type (say,
   <command>INSERT</command>) will be fired whenever we
   <emphasis>specify</emphasis> an action of that kind.
   In contrast, row-level triggers will fire only for the specific event type
   being <emphasis>executed</emphasis>.
   So a <command>MERGE</command> command might fire statement triggers for both
   <command>UPDATE</command> and <command>INSERT</command>, even though only
   <command>UPDATE</command> row triggers were fired.
  </para>

  <para>
   You should ensure that the join produces at most one candidate change row
   for each target row.  In other words, a target row shouldn't join to more
   than one data source row.  If it does, then only one of the candidate change
   rows will be used to modify the target row; later attempts to modify the
   row will cause an error.
   This can also occur if row triggers make changes to the target table
   and the rows so modified are then subsequently also modified by
   <command>MERGE</command>.
   If the repeated action is an <command>INSERT</command>, this will
   cause a uniqueness violation, while a repeated <command>UPDATE</command>
   or <command>DELETE</command> will cause a cardinality violation; the
   latter behavior is required by the <acronym>SQL</acronym> standard.
   This differs from historical <productname>PostgreSQL</productname>
   behavior of joins in <command>UPDATE</command> and
   <command>DELETE</command> statements where second and subsequent
   attempts to modify the same row are simply ignored.
  </para>

  <para>
   If a <literal>WHEN</literal> clause omits an <literal>AND</literal>
   sub-clause, it becomes the final reachable clause of that
   kind (<literal>MATCHED</literal>, <literal>NOT MATCHED BY SOURCE</literal>,
   or <literal>NOT MATCHED [BY TARGET]</literal>).
   If a later <literal>WHEN</literal> clause of that kind
   is specified it would be provably unreachable and an error is raised.
   If no final reachable clause is specified of either kind, it is
   possible that no action will be taken for a candidate change row.
  </para>

  <para>
   The order in which rows are generated from the data source is
   indeterminate by default.
   A <replaceable class="parameter">source_query</replaceable> can be
   used to specify a consistent ordering, if required, which might be
   needed to avoid deadlocks between concurrent transactions.
  </para>

  <para>
   When <command>MERGE</command> is run concurrently with other commands
   that modify the target table, the usual transaction isolation rules
   apply; see <xref linkend="transaction-iso"/> for an explanation
   on the behavior at each isolation level.
   You may also wish to consider using <command>INSERT ... ON CONFLICT</command>
   as an alternative statement which offers the ability to run an
   <command>UPDATE</command> if a concurrent <command>INSERT</command>
   occurs.  There are a variety of differences and restrictions between
   the two statement types and they are not interchangeable.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Perform maintenance on <literal>customer_accounts</literal> based
   upon new <literal>recent_transactions</literal>.

<programlisting>
MERGE INTO customer_account ca
USING recent_transactions t
ON t.customer_id = ca.customer_id
WHEN MATCHED THEN
  UPDATE SET balance = balance + transaction_value
WHEN NOT MATCHED THEN
  INSERT (customer_id, balance)
  VALUES (t.customer_id, t.transaction_value);
</programlisting>
  </para>

  <para>
   Attempt to insert a new stock item along with the quantity of stock. If
   the item already exists, instead update the stock count of the existing
   item. Don't allow entries that have zero stock. Return details of all
   changes made.
<programlisting>
MERGE INTO wines w
USING wine_stock_changes s
ON s.winename = w.winename
WHEN NOT MATCHED AND s.stock_delta > 0 THEN
  INSERT VALUES(s.winename, s.stock_delta)
WHEN MATCHED AND w.stock + s.stock_delta > 0 THEN
  UPDATE SET stock = w.stock + s.stock_delta
WHEN MATCHED THEN
  DELETE
RETURNING merge_action(), w.winename, old.stock AS old_stock, new.stock AS new_stock;
</programlisting>

   The <literal>wine_stock_changes</literal> table might be, for example, a
   temporary table recently loaded into the database.
  </para>

  <para>
   Update <literal>wines</literal> based on a replacement wine list, inserting
   rows for any new stock, updating modified stock entries, and deleting any
   wines not present in the new list.
<programlisting>
MERGE INTO wines w
USING new_wine_list s
ON s.winename = w.winename
WHEN NOT MATCHED BY TARGET THEN
  INSERT VALUES(s.winename, s.stock)
WHEN MATCHED AND w.stock != s.stock THEN
  UPDATE SET stock = s.stock
WHEN NOT MATCHED BY SOURCE THEN
  DELETE;
</programlisting>
  </para>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>
   <para>
    This command conforms to the <acronym>SQL</acronym> standard.
  </para>
   <para>
    The <literal>WITH</literal> clause, <literal>BY SOURCE</literal> and
    <literal>BY TARGET</literal> qualifiers to
    <literal>WHEN NOT MATCHED</literal>, <literal>DO NOTHING</literal> action,
    and <literal>RETURNING</literal> clause are extensions to the
    <acronym>SQL</acronym> standard.
  </para>
 </refsect1>
</refentry>
