SELECT from DELETE, UPDATE, and MERGE [DB2 9 for z/OS]

Another nice new SQL feature provides the ability to SELECT from DELETE, UPDATE, and MERGE statements.

This capability is similar to the SELECT from INSERT feature that was introduced with DB2 V8. So, before looking at the new V9 feature, let’s review the V8 feature.

The ability to SELECT from an INSERT statement is an intriguing feature. To understand why this is so, we need to review some background details. In some cases, it is possible to perform actions on an inserted row before it gets saved to disk. For example, a BEFORE TRIGGER might change data before it is even recorded to disk. But the application program will not have any knowledge of this change that is made in the trigger. Identity columns and columns with defaults (especially user-defined defaults) have similar effects. What if the program needs to know the final column values? Prior to V8 this was difficult and inefficient to implement.

Well, the SELECT FROM INSERT feature introduced in DB2 V8 solves this problem. It allows you to both insert the row and retrieve the values of the columns with a single SQL statement. It performs very well because it performs both the INSERT and the SELECT as a single operation. Consider the following example:

  SELECT COL5 INTO :C5-HV
  FROM FINAL TABLE
   (INSERT (COL1, COL2, COL5, COL7) INTO SAMPLE_TABLE
    VALUES('JONES', 'CHARLES', CURRENT DATE, 'HOURLY')
   );

The data is inserted as specified in the VALUES clause, and retrieved as specified in the SELECT. Without the ability to select COL5, the program would have no knowledge of the value supplied to COL5, because it was assigned using CURRENT DATE. With this new syntax the program can retrieve the CURRENT DATE value that was just inserted into COL5 without incurring additional overhead.

OK, on to V9. In this new version you can retrieve columns from rows that are modified via DELETE, UPDATE, and MERGE statements, thereby replacing multiple SQL calls with one. DB2 V9 allows a searched UPDATE or a searched DELETE statement in the FROM clause of a SELECT statement that is a subselect, or in the SELECT INTO statement. This capability allows a user or program to know which values were updated or deleted.

And, if you recall from a few blog entries ago, DB2 9 for z/OS also adds the MERGE statement. Well, it also adds the ability to SELECT from the MERGE. This allows you to return all the rows that were either inserted or updated as a result of the MERGE.

Here is an example of a SELECT from an UPDATE:

 SELECT SUM(salary) INTO :SAL-HV
 FROM FINAL_TABLE
  (UPDATE EMP
   SET SALARY = SALARY * 1.02
   WHERE WORKDEPT = ‘A01’);

Prior to the capability you would have had to run the UPDATE statement, and then only after it finishes, you would run the SELECT to add up the new salary values. Now, instead of multiple statements requiring multiple passes through the data, you can consolidate it into one.

Nice, huh?