Description

MERGE performs actions that modify rows in the target table identified as 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 the target table 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, 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 that are referred to in the SET clause. If you specify an insert action, you must have the INSERT privilege on the target table. If you specify a delete action, you must have the DELETE privilege on the target table. If you specify a DO NOTHING action, you must have the SELECT privilege on at least one column of the target table. You will also need SELECT privilege on any column(s) of the data_source and of the target table referred to in any condition (including join_condition) or expression. Privileges are tested once at statement start and are checked whether or not particular WHEN clauses are executed.

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

Parameters

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.

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.