9.4. The MERGE Statement

A MERGE statement combines an INSERT statement with an UPDATE or DELETE statement. For example, if a row in table T1 also exists in table T2, the existing row in T2 should be updated. If a row in T1 does not exist in T2, it should be inserted into T2. A new and efficient way to code this logic can be implemented with one statement: the MERGE statement. Figure 9.25 shows this MERGE statement.

Figure 9.25. Example of a MERGE statement
					MERGE INTO T2 as target
					USING (SELECT ... FROM T1) AS source
					ON target.id=source.id
					WHEN NOT MATCHED THEN
					INSERT INTO T2 ...
					WHEN MATCHED THEN
					UPDATE T2 SET ...

Figure 9.26 illustrates the syntax of a MERGE statement. The MERGE statement has a lot of intricate details; see the DB2 UDB SQL Reference manual for more examples and additional information.

Figure 9.26. Syntax of the MERGE statement
>>-MERGE INTO--+-table-name-------+----------------------------->
               +-view-name--------+
               '-(--fullselect--)-'

>--+------------------------+--USING--table-reference----------->
   '-| correlation-clause |-'

>--ON--search-condition----------------------------------------->

   .----------------------------------------------------------------.
   V                                                                |
>----WHEN--| matching-condition |--THEN--+-| modification-operation |+>
                                         '-signal-statement---------'

   .-ELSE IGNORE-.
>--+-------------+---------------------------------------------><

NOTE

Refer to Chapter 1, Introduction to DB2 UDB, for a description of the DB2 syntax diagram conventions.


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