6.6 MERGE

Used forMerging data from a source set into a target relation

Available inDSQL, PSQL

Syntax

MERGE INTO target [[AS] target_alias]
  USING <source> [[AS] source_alias]
  ON <join_condition>
  <merge_when> [<merge_when> ...]
  [RETURNING <returning_list> [INTO <variables>]]
 
<merge_when> ::=
    <merge_when_matched>
  | <merge_when_not_matched>
 
<merge_when_matched> ::=
  WHEN MATCHED [ AND <condition> ] THEN
  { UPDATE SET <assignment-list>
  | DELETE }
 
<merge_when_not_matched> ::=
  WHEN NOT MATCHED [ AND <condition> ] THEN
  INSERT [( <column_list> )] [<override_opt>]
  VALUES ( <value_list> )
 
<source> ::= tablename | (<select_stmt>)
 
<assignment_list ::=
  col_name = <m_value> [, <col_name> = <m_value> ...]]
 
<override_opt> ::=
  OVERRIDING {USER | SYSTEM} VALUE
 
<column_list> ::= colname [, colname ...]
 
<value_list> ::= <m_value> [, <m_value> ...]
 
<m_value> ::= <value_expression> | DEFAULT
 
<returning_list> ::= * | <output_column> [, <output_column]
 
<output_column> ::=
    target.* | NEW.* | OLD.*
  | <return_expression> [COLLATE collation] [[AS] alias]
 
<return_expression> ::=
    <value_expression>
  | [target.]col_name
  | NEW.col_name
  | OLD.col_name
 
<value_expression> ::=
    <literal>
  | <context-variable>
  | any other expression returning a single
    value of a Firebird data type or NULL
 
<variables> ::=
  [:]varname [, [:]varname ...]

Table 6.6.1 Arguments for the MERGE Statement Parameters

The MERGE statement merges records from the source into a target table or updatable view. The source may be a table, view or anything you can SELECT from in general. Each source record will be used to update one or more target records, insert a new record in the target table, delete a record from the target table or do nothing.

The action taken depends on the supplied join condition, the WHEN clause(s), and the - optional - condition in the WHEN clause. The join condition and condition in the WHEN will typically contain a comparison of fields in the source and target relations.

Multiple WHEN MATCHED and WHEN NOT MATCHED clauses are allowed. For each row in the source, the WHEN clauses are checked in the order they are specified in the statement. If the condition in the WHEN clause does not evaluate to true, the clause is skipped, and the next clause will be checked. This will be done until the condition for a WHEN clause evaluates to true, or a WHEN clauses without condition matches, or there are no more WHEN clauses. If a matching clause is found, the action associated with the clause is executed. For each row in the source, at most one action is executed. If the WHEN MATCHED clause is present, and several records match a single record in the target table, an error is raised.

Warning

At least one WHEN clause must be present.

WHEN NOT MATCHED is evaluated from the source viewpoint, that is, the table or set specified in USING. It has to work this way because if the source record does not match a target record, INSERT is executed. Of course, if there is a target record which does not match a source record, nothing is done.

Currently, the ROW_COUNT variable returns the value 1, even if more than one record is modified or inserted. For details and progress, refer to firebird#4722.

6.6.1 The RETURNING Clause

A MERGE statement that affects at most one row can contain a RETURNING clause to return values added, modified or removed. If a RETURNING clause is present and more than one matching record is found, an error multiple rows in singleton select is raised. The RETURNING clause can contain any columns from the target table (or updateable 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.

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

Note

The restriction that RETURNING can only be used with a statement that affects at most one row might be removed in a future version.

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

6.6.2 Examples of MERGE

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