NAME

       MERGE - conditionally insert, update, or delete rows of a table

SYNOPSIS

       [ WITH with_query [, ...] ]
       MERGE INTO [ ONLY ] target_table_name [ * ] [ [ AS ] target_alias ]
       USING data_source ON join_condition
       when_clause [...]

       where data_source is:

       { [ ONLY ] source_table_name [ * ] | ( source_query ) } [ [ AS ] source_alias ]

       and when_clause is:

       { WHEN MATCHED [ AND condition ] THEN { merge_update | merge_delete | DO NOTHING } |
         WHEN NOT MATCHED [ AND condition ] THEN { merge_insert | DO NOTHING } }

       and merge_insert is:

       INSERT [( column_name [, ...] )]
       [ OVERRIDING { SYSTEM | USER } VALUE ]
       { VALUES ( { expression | DEFAULT } [, ...] ) | DEFAULT VALUES }

       and merge_update is:

       UPDATE SET { column_name = { expression | DEFAULT } |
                    ( column_name [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...]

       and merge_delete is:

       DELETE

DESCRIPTION

       MERGE performs actions that modify rows in the target_table_name, using the data_source.
       MERGE provides a single SQL statement that can conditionally INSERT, UPDATE or DELETE
       rows, a task that would otherwise require multiple procedural language statements.

       First, the MERGE command performs a join from data_source to target_table_name producing
       zero or more candidate change rows. For each candidate change row, the status of MATCHED
       or NOT MATCHED is set just once, after which WHEN 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 WHEN clause is executed for any candidate change row.

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

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

       There is no separate MERGE privilege. If you specify an update action, you must have the
       UPDATE privilege on the column(s) of the target_table_name that are referred to in the SET
       clause. If you specify an insert action, you must have the INSERT privilege on the
       target_table_name. If you specify an delete action, you must have the DELETE privilege on
       the target_table_name. Privileges are tested once at statement start and are checked
       whether or not particular WHEN clauses are executed. You will require the SELECT privilege
       on the data_source and any column(s) of the target_table_name referred to in a condition.

       MERGE is not supported if the target_table_name is a materialized view, foreign table, or
       if it has any rules defined on it.

PARAMETERS

       target_table_name
           The name (optionally schema-qualified) of the target table to merge into. If ONLY is
           specified before the table name, matching rows are updated or deleted in the named
           table only. If ONLY is not specified, matching rows are also updated or deleted in any
           tables inheriting from the named table. Optionally, * can be specified after the table
           name to explicitly indicate that descendant tables are included. The ONLY keyword and
           * option do not affect insert actions, which always insert into the named table only.

       target_alias
           A substitute name for the target table. When an alias is provided, it completely hides
           the actual name of the table. For example, given MERGE INTO foo AS f, the remainder of
           the MERGE statement must refer to this table as f not foo.

       source_table_name
           The name (optionally schema-qualified) of the source table, view, or transition table.
           If ONLY is specified before the table name, matching rows are included from the named
           table only. If ONLY is not specified, matching rows are also included from any tables
           inheriting from the named table. Optionally, * can be specified after the table name
           to explicitly indicate that descendant tables are included.

       source_query
           A query (SELECT statement or VALUES statement) that supplies the rows to be merged
           into the target_table_name. Refer to the SELECT(7) statement or VALUES(7) statement
           for a description of the syntax.

       source_alias
           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.

       join_condition
           join_condition is an expression resulting in a value of type boolean (similar to a
           WHERE clause) that specifies which rows in the data_source match rows in the
           target_table_name.

               Warning
               Only columns from target_table_name that attempt to match data_source rows should
               appear in join_condition.  join_condition subexpressions that only reference
               target_table_name columns can affect which action is taken, often in surprising
               ways.

       when_clause
           At least one WHEN clause is required.

           If the WHEN clause specifies WHEN MATCHED and the candidate change row matches a row
           in the target_table_name, the WHEN clause is executed if the condition is absent or it
           evaluates to true.

           Conversely, if the WHEN clause specifies WHEN NOT MATCHED and the candidate change row
           does not match a row in the target_table_name, the WHEN clause is executed if the
           condition is absent or it evaluates to true.

       condition
           An expression that returns a value of type boolean. If this expression for a WHEN
           clause returns true, then the action for that clause is executed for that row.

           A condition on a WHEN MATCHED clause can refer to columns in both the source and the
           target relations. A condition on a WHEN NOT MATCHED 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.

       merge_insert
           The specification of an INSERT 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.

           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.

           If target_table_name is a partitioned table, each row is routed to the appropriate
           partition and inserted into it. If target_table_name is a partition, an error will
           occur if any input row violates the partition constraint.

           Column names may not be specified more than once.  INSERT actions cannot contain
           sub-selects.

           Only one VALUES clause can be specified. The VALUES clause can only refer to columns
           from the source relation, since by definition there is no matching target row.

       merge_update
           The specification of an UPDATE action that updates the current row of the
           target_table_name. Column names may not be specified more than once.

           Neither a table name nor a WHERE clause are allowed.

       merge_delete
           Specifies a DELETE action that deletes the current row of the target_table_name. Do
           not include the table name or any other clauses, as you would normally do with a
           DELETE(7) command.

       column_name
           The name of a column in the target_table_name. 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.

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

       OVERRIDING USER VALUE
           If this clause is specified, then any values supplied for identity columns defined as
           GENERATED BY DEFAULT are ignored and the default sequence-generated values are
           applied.

       DEFAULT VALUES
           All columns will be filled with their default values. (An OVERRIDING clause is not
           permitted in this form.)

       expression
           An expression to assign to the column. If used in a WHEN MATCHED clause, the
           expression can use values from the original row in the target table, and values from
           the data_source row. If used in a WHEN NOT MATCHED clause, the expression can use
           values from the data_source.

       DEFAULT
           Set the column to its default value (which will be NULL if no specific default
           expression has been assigned to it).

       with_query
           The WITH clause allows you to specify one or more subqueries that can be referenced by
           name in the MERGE query. See Section 7.8 and SELECT(7) for details.

OUTPUTS

       On successful completion, a MERGE command returns a command tag of the form

           MERGE total_count

       The total_count is the total number of rows changed (whether inserted, updated, or
       deleted). If total_count is 0, no rows were changed in any way.

NOTES

       The following steps take place during the execution of MERGE.

        1. Perform any BEFORE STATEMENT triggers for all actions specified, whether or not their
           WHEN clauses match.

        2. 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,

            1. Evaluate whether each row is MATCHED or NOT MATCHED.

            2. Test each WHEN condition in the order specified until one returns true.

            3. When a condition returns true, perform the following actions:

                1. Perform any BEFORE ROW triggers that fire for the action's event type.

                2. Perform the specified action, invoking any check constraints on the target
                   table.

                3. Perform any AFTER ROW triggers that fire for the action's event type.

        3. Perform any AFTER STATEMENT triggers for actions specified, whether or not they
           actually occur. This is similar to the behavior of an UPDATE statement that modifies
           no rows.

       In summary, statement triggers for an event type (say, INSERT) will be fired whenever we
       specify an action of that kind. In contrast, row-level triggers will fire only for the
       specific event type being executed. So a MERGE command might fire statement triggers for
       both UPDATE and INSERT, even though only UPDATE row triggers were fired.

       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 MERGE. If the repeated action is an INSERT, this will cause a uniqueness
       violation, while a repeated UPDATE or DELETE will cause a cardinality violation; the
       latter behavior is required by the SQL standard. This differs from historical PostgreSQL
       behavior of joins in UPDATE and DELETE statements where second and subsequent attempts to
       modify the same row are simply ignored.

       If a WHEN clause omits an AND sub-clause, it becomes the final reachable clause of that
       kind (MATCHED or NOT MATCHED). If a later WHEN 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.

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

       There is no RETURNING clause with MERGE. Actions of INSERT, UPDATE and DELETE cannot
       contain RETURNING or WITH clauses.

       When MERGE is run concurrently with other commands that modify the target table, the usual
       transaction isolation rules apply; see Section 13.2 for an explanation on the behavior at
       each isolation level. You may also wish to consider using INSERT ... ON CONFLICT as an
       alternative statement which offers the ability to run an UPDATE if a concurrent INSERT
       occurs. There are a variety of differences and restrictions between the two statement
       types and they are not interchangeable.

EXAMPLES

       Perform maintenance on customer_accounts based upon new recent_transactions.

           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);

       Notice that this would be exactly equivalent to the following statement because the
       MATCHED result does not change during execution.

           MERGE INTO customer_account ca
           USING (SELECT customer_id, transaction_value FROM recent_transactions) AS 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);

       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.

           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;

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

COMPATIBILITY

       This command conforms to the SQL standard.

       The WITH clause and DO NOTHING action are extensions to the SQL standard.