The ORDER BY can be used to influence the order in which rows are evaluated. The primary use case is when combined with RETURNING, to influence the order rows are returned.

A MERGE statement can contain a RETURNING clause to return rows added, modified or removed. The merge is executed to completion before rows are returned. The RETURNING clause can contain any columns from the target table (or updatable view), as well as other columns (eg from the source) and expressions.

The user executing the statement needs to have SELECT privileges on the columns specified in the RETURNING clause.

In PSQL, If a RETURNING clause is present and more than one matching record is found, an error multiple rows in singleton select is raised. This behaviour may change in a future Firebird version.

The optional INTO sub-clause is only valid in PSQL.

Column names can be qualified by the OLD or NEW prefix to define exactly what value to return: before or after modification. The returned values include the changes made by BEFORE triggers.

The syntax of the returning_list is similar to the column list of a SELECT clause. It is possible to reference all columns using *, or table_name.*, NEW.* and/or OLD.*.

For the UPDATE or INSERT action, unqualified column names, or those qualified by the target table name or alias will behave as if qualified by NEW, while for the DELETE action as if qualified by OLD.

The following example modifies the previous example to affect one line, and adds a RETURNING clause to return the old and new quantity of goods, and the difference between those values.

Using MERGE with a RETURNING clause

MERGE INTO PRODUCT_INVENTORY AS TARGET
USING (
  SELECT
    SL.ID_PRODUCT,
    SUM(SL.QUANTITY)
  FROM SALES_ORDER_LINE SL
  JOIN SALES_ORDER S ON S.ID = SL.ID_SALES_ORDER
  WHERE S.BYDATE = CURRENT_DATE
  AND SL.ID_PRODUCT =: ID_PRODUCT
  GROUP BY 1
) AS SRC (ID_PRODUCT, QUANTITY)
ON TARGET.ID_PRODUCT = SRC.ID_PRODUCT
WHEN MATCHED AND TARGET.QUANTITY - SRC.QUANTITY <= 0 THEN
  DELETE
WHEN MATCHED THEN
  UPDATE SET
    TARGET.QUANTITY = TARGET.QUANTITY - SRC.QUANTITY,
    TARGET.BYDATE = CURRENT_DATE
RETURNING OLD.QUANTITY, NEW.QUANTITY, SRC.QUANTITY
INTO : OLD_QUANTITY, :NEW_QUANTITY, :DIFF_QUANTITY

1. Update books when present, or add new record if absent

   MERGE INTO books b
     USING purchases p
     ON p.title = b.title and p.type = 'bk'
     WHEN MATCHED THEN
       UPDATE SET b.desc = b.desc || '; ' || p.desc
     WHEN NOT MATCHED THEN
       INSERT (title, desc, bought) values (p.title, p.desc, p.bought);

2. Using a derived table

   MERGE INTO customers c
     USING (SELECT * from customers_delta WHERE id > 10) cd
     ON (c.id = cd.id)
     WHEN MATCHED THEN
       UPDATE SET name = cd.name
     WHEN NOT MATCHED THEN
       INSERT (id, name) values (cd.id, cd.name);

3. Together with a recursive CTE

   MERGE INTO numbers
     USING (
       WITH RECURSIVE r(n) AS (
         SELECT 1 FROM rdb$database
         UNION ALL
         SELECT n+1 FROM r WHERE n < 200
       )
       SELECT n FROM r
     ) t
     ON numbers.num = t.n
     WHEN NOT MATCHED THEN
       INSERT(num) VALUES(t.n);

4. Using DELETE clause

   MERGE INTO SALARY_HISTORY
   USING (
     SELECT EMP_NO
     FROM EMPLOYEE
     WHERE DEPT_NO = 120) EMP
   ON SALARY_HISTORY.EMP_NO = EMP.EMP_NO
   WHEN MATCHED THEN DELETE

5. The following example updates the PRODUCT_INVENTORY table daily based on orders processed in the SALES_ORDER_LINE table. If the stock level of the product would drop to zero or lower, then the row for that product is removed from the PRODUCT_INVENTORY table.

   MERGE INTO PRODUCT_INVENTORY AS TARGET
   USING (
     SELECT
       SL.ID_PRODUCT,
       SUM (SL.QUANTITY)
     FROM SALES_ORDER_LINE SL
     JOIN SALES_ORDER S ON S.ID = SL.ID_SALES_ORDER
     WHERE S.BYDATE = CURRENT_DATE
     GROUP BY 1
   ) AS SRC (ID_PRODUCT, QUANTITY)
   ON TARGET.ID_PRODUCT = SRC.ID_PRODUCT
   WHEN MATCHED AND TARGET.QUANTITY - SRC.QUANTITY <= 0 THEN
     DELETE
   WHEN MATCHED THEN
     UPDATE SET
       TARGET.QUANTITY = TARGET.QUANTITY - SRC.QUANTITY,
       TARGET.BYDATE = CURRENT_DATE

See alsoSection 6.1, “SELECT”, Section 6.2, “INSERT”, Section 6.3, “UPDATE”, Section 6.4, “UPDATE OR INSERT”, Section 6.5, “DELETE”