<!--
doc/src/sgml/ref/merge.sgml
PostgreSQL documentation
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<refentry id="sql-merge">

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

 <refnamediv>
  <refname>MERGE</refname>
  <refpurpose>insert, update, or delete rows of a table based upon source data</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
[ WITH <replaceable class="parameter">with_query</replaceable> [, ...] ]
MERGE INTO <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> [...]

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

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

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

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

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

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

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

UPDATE SET { <replaceable class="parameter">column_name</replaceable> = { <replaceable class="parameter">expression</replaceable> | DEFAULT } |
             ( <replaceable class="parameter">column_name</replaceable> [, ...] ) = ( { <replaceable class="parameter">expression</replaceable> | DEFAULT } [, ...] )
           } [, ...]

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

DELETE
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>MERGE</command> performs actions that modify rows in the
   <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
   <replaceable class="parameter">target_table_name</replaceable>
   producing zero or more candidate change rows.  For each candidate change
   row the status of <literal>MATCHED</literal> or <literal>NOT MATCHED</literal> is set
   just once, after which <literal>WHEN</literal> clauses are evaluated
   in the order specified. If one of them is activated, the specified
   action occurs. No more than one <literal>WHEN</literal> clause can be
   activated 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 tablename is specified.  All actions refer to the
   <replaceable class="parameter">target_table_name</replaceable>,
   though modifications to other tables may be made using triggers.
  </para>

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

  <para>
   There is no MERGE privilege.  
   You must have the <literal>UPDATE</literal> privilege on the column(s)
   of the <replaceable class="parameter">target_table_name</replaceable>
   referred to in the <literal>SET</literal> clause
   if you specify an update action, the <literal>INSERT</literal> privilege
   on the <replaceable class="parameter">target_table_name</replaceable>
   if you specify an insert action and/or the <literal>DELETE</literal>
   privilege on the <replaceable class="parameter">target_table_name</replaceable>
   if you specify a delete action on the
   <replaceable class="parameter">target_table_name</replaceable>.
   Privileges are tested once at statement start and are checked
   whether or not particular <literal>WHEN</literal> clauses are activated
   during the subsequent execution.
   You will require the <literal>SELECT</literal> privilege on the
   <replaceable class="parameter">data_source</replaceable> and any column(s)
   of the <replaceable class="parameter">target_table_name</replaceable>
   referred to in a <literal>condition</literal>.
  </para>

  <para>
   MERGE is not supported if the <replaceable
   class="parameter">target_table_name</replaceable> has
   <literal>RULES</literal> defined on it.
   See <xref linkend="rules"/> for more information about <literal>RULES</literal>.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">target_table_name</replaceable></term>
    <listitem>
     <para>
      The name (optionally schema-qualified) of the target table to merge into.
     </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 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.
     </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
      <replaceable class="parameter">target_table_name</replaceable>.
      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 whether table or query was specified.
     </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
      <replaceable class="parameter">target_table_name</replaceable>.
     </para>
     <warning>
      <para>
       Only columns from <replaceable class="parameter">target_table_name</replaceable>
       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 <replaceable class="parameter">target_table_name</replaceable>
       columns can only affect which action is taken, often in surprising ways.
      </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>
      If the <literal>WHEN</literal> clause specifies <literal>WHEN MATCHED</literal>
      and the candidate change row matches a row in the
      <replaceable class="parameter">target_table_name</replaceable>
      the <literal>WHEN</literal> clause is activated if the
      <replaceable class="parameter">condition</replaceable> is
      absent or is present and evaluates to <literal>true</literal>.
      If the <literal>WHEN</literal> clause specifies <literal>WHEN NOT MATCHED</literal>
      and the candidate change row does not match a row in the
      <replaceable class="parameter">target_table_name</replaceable>
      the <literal>WHEN</literal> clause is activated if the
      <replaceable class="parameter">condition</replaceable> is
      absent or is present and 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 returns <literal>true</literal> then the <literal>WHEN</literal>
      clause will be activated and the corresponding action will occur for
      that row. The expression may not contain functions that possibly performs
      writes to the database.
     </para>
     <para>
      A condition on a <literal>WHEN MATCHED</literal> clause can refer to columns
      in both the source and the target relation. A condition on a
      <literal>WHEN NOT MATCHED</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 expression for any column is not of the correct data type,
      automatic type conversion will be attempted.
     </para>
     <para>
      If <replaceable class="parameter">target_table_name</replaceable>
      is a partitioned table, each row is routed to the appropriate partition
      and inserted into it.
      If <replaceable class="parameter">target_table_name</replaceable>
      is a partition, an error will occur if one of the input rows 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>
      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 <replaceable
      class="parameter">target_table_name</replaceable>.
      Column names may not be specified more than once.
     </para>
     <para>
      Do not include the table name, as you would normally do with an
      <xref linkend="sql-update"/> command.
      For example, <literal>UPDATE tab SET col = 1</literal> is invalid. Also,
      do not include a <literal>WHERE</literal> clause, since only the current
      row can be updated. For example,
      <literal>UPDATE SET col = 1 WHERE key = 57</literal> is invalid.
     </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 <replaceable class="parameter">target_table_name</replaceable>.
      Do not include the tablename or any other clauses, as you would normally
      do with an <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 <replaceable
      class="parameter">target_table_name</replaceable>.  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.)  When referencing a
      column, 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.  The expression can use the
      old values of this and other columns in the table.
     </para>
    </listitem>
   </varlistentry>

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

   <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.
     </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>

 </refsect1>

 <refsect1>
  <title>Execution</title>

  <para>
   The following steps take place during the execution of
   <command>MERGE</command>.
    <orderedlist>
     <listitem>
      <para>
       Perform any BEFORE STATEMENT triggers for all actions specified, whether or
       not their <literal>WHEN</literal> clauses are activated during execution.
      </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 row. For each candidate change row
       <orderedlist>
        <listitem>
         <para>
          Evaluate whether each row is MATCHED or NOT MATCHED.
         </para>
        </listitem>
        <listitem>
         <para>
          Test each WHEN condition in the order specified until one activates.
         </para>
        </listitem>
        <listitem>
         <para>
          When activated, perform the following actions
          <orderedlist>
           <listitem>
            <para>
             Perform any BEFORE ROW triggers that fire for the action's event type.
            </para>
           </listitem>
           <listitem>
            <para>
             Apply the action specified, invoking any check constraints on the
             target table.
             However, it will not invoke rules.
            </para>
           </listitem>
           <listitem>
            <para>
             Perform any AFTER ROW triggers that fire for the action's event type.
            </para>
           </listitem>
          </orderedlist>
         </para>
        </listitem>
       </orderedlist>
      </para>
     </listitem>
     <listitem>
      <para>
       Perform any AFTER STATEMENT 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, INSERT) will
   be fired whenever we <emphasis>specify</emphasis> an action of that kind. Row-level
   triggers will fire only for the one event type <emphasis>activated</emphasis>.
   So a <command>MERGE</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 will
   cause an error.  This can also occur if row triggers make changes to the
   target table which are then subsequently 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 are simply ignored.
  </para>

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

 </refsect1>
 <refsect1>
  <title>Notes</title>

  <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>
   There is no <literal>RETURNING</literal> clause with <command>MERGE</command>.
   Actions of <command>INSERT</command>, <command>UPDATE</command> and <command>DELETE</command>
   cannot contain <literal>RETURNING</literal> or <literal>WITH</literal> clauses.
  </para>

  <tip>
   <para>
    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>
  </tip>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Perform maintenance on CustomerAccounts based upon new Transactions.

<programlisting>
MERGE CustomerAccount CA
USING RecentTransactions T
ON T.CustomerId = CA.CustomerId
WHEN MATCHED THEN
  UPDATE SET Balance = Balance + TransactionValue
WHEN NOT MATCHED THEN
  INSERT (CustomerId, Balance)
  VALUES (T.CustomerId, T.TransactionValue);
</programlisting>

   notice that this would be exactly equivalent to the following
   statement because the <literal>MATCHED</literal> result does not change
   during execution

<programlisting>
MERGE CustomerAccount CA
USING (Select CustomerId, TransactionValue From RecentTransactions) AS T
ON CA.CustomerId = T.CustomerId
WHEN NOT MATCHED THEN
  INSERT (CustomerId, Balance)
  VALUES (T.CustomerId, T.TransactionValue)
WHEN MATCHED THEN
  UPDATE SET Balance = Balance + TransactionValue;
</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.
<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;
</programlisting>

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

 </refsect1>

 <refsect1>
  <title>Compatibility</title>
   <para>
    This command conforms to the <acronym>SQL</acronym> standard.
  </para>
   <para>
    The WITH clause and DO NOTHING action are extensions to the <acronym>SQL</acronym> standard.
  </para>
 </refsect1>
</refentry>
