INSTEAD OF Triggers [DB2 9 for z/OS]
DB2 9 for z/OS introduces a new type of trigger: the INSTEAD OF trigger. The primary usage of INSTEAD OF triggers is to enable views that would not otherwise be updatable to support updates. Typically, a view that consists of multiple base tables cannot be updated. But with an INSTEAD OF trigger this problem can be surmounted. You can code an INSTEAD OF trigger to direct inserts, updates and deletes to the appropriate underlying tables of the view.
With the INSTEAD OF trigger, your application code does not have to include complex algorithms to specify which operations should be performed against views and which should be performed against base table. Instead, all actions are performed against the view and the activated trigger determines which underlying base tables are to be impacted.
So, you might choose to code an INSTEAD OF trigger on a view over a join to allow modifications on the view to go through to the underlying base tables joined in that view. Or you can encode and decode data within a view: for example, the view could contain the decryption functions while the INSTEAD OF triggers use the encryption functions to ensure security.
Only one INSTEAD OF trigger is allowed for each type of operation on a given subject view. That is, one for inserts, one for updates, and one for deletes. Therefore, you can have a grand total of three INSTEAD OF triggers per view.
DB2 executes the triggered-action instead of the insert, update, or delete operation on the subject view. Neither the WHEN clause nor the FOR EACH STATEMENT clause are allowed in INSTEAD OF triggers.
Furthermore, there are some restrictions on the view in order for an INSTEAD OF trigger to be allowed. First of all, the view must exist at the current server when the INSTEAD OF trigger is created. Additionally, none of the following are permitted for a view to have an INSTEAD OF trigger:
- the WITH CASCADED CHECK option
- a view on which a symmetric view has been defined
- a view that references data encoded with different encoding schemes or CCSID values
- a view with a ROWID, LOB, or XML column (or a distinct type that is defined as one of these types)
- a view with a column based on an underlying column defined as an identity column, security label column, or a row change timestamp column
- a with a column that is defined (directly or indirectly) as an expression
- a view with a column that is based on a column of a result table that involves a set operator
- a view with any columns that have field procedures
- a view where all of the underlying base tables are DB2 Catalog tables or created global temporary tables
- a view that has other views dependent on it
One way to think of INSTEAD OF triggers is that they contain the inverse of the logic in the body of the view. If the view joins tables, the trigger should break the join apart to modify the correct data. If the view decrypts columns, the INSTEAD OF trigger should encrypt the columns. Etc.
Let’s take a look at an example to better understand the INSTEAD OF trigger. First, we create a view that joins the EMP and DEPT tables:
CREATE VIEW EMP_DEPT (EMPNO, FIRSTNME, MIDINIT, LASTNAME,
PHONENO, HIREDATE, DEPTNAME)
AS SELECT EMPNO, FIRSTNME, MIDINIT, LASTNAME, PHONENO,
FROM EMP, DEPT
WHERE EMP.WORKDEPT = DEPT.DEPTNO;
OK, so far, so good. But since this view is a join it is not updateable. Let’s fix this by coding up some INSTEAD OF triggers. First, we’ll take care of inserts:
CREATE TRIGGER E_D_ISRT
INSTEAD OF INSERT ON EMP_DEPT
REFERENCING NEW AS NEWEMP
FOR EACH ROW INSERT INTO EMPLOYEE
(EMPNO, FIRSTNME, MIDINIT, LASTNAME, WORKDEPT, PHONENO, HIREDATE)
(EMPNO, FIRSTNME, MIDINIT, LASTNAME,
((SELECT DEPTNO FROM DEPT AS D WHERE D.DEPTNAME = NEWEMP.DEPTNAME),
RAISE_ERROR('70001', 'Unknown dept name')
An insert against the view would not be inserting a new department, so we will be inserting data into the EMP table. If the department does not exist, we’ll raise an error. Next we’ll consider updates:
CREATE TRIGGER E_D_UPD
INSTEAD OF UPDATE ON EMP_DEPT
REFERENCING NEW AS NEWEMP OLD AS OLDEMP
FOR EACH ROW
VALUES(CASE WHEN NEWEMP.EMPNO = OLDEMP.EMPNO THEN 0
ELSE RAISE_ERROR('70002', 'Must not change EMPNO') END);
UPDATE EMP AS E SET
(FIRSTNME, MIDINIT, LASTNAME, WORKDEPT, PHONENO, HIREDATE)
= (NEWEMP.FIRSTNME, NEWEMP.MIDINIT, NEWEMP.LASTNAME,
COALESCE((SELECT DEPTNO FROM DEPT AS D
WHERE D.DEPTNAME = NEWEMP.DEPTNAME),
RAISE_ERROR ('70001', 'Unknown dept name')),
WHERE NEWEMP.EMPNO = E.EMPNO;
Finally we take care of deletions:
CREATE TRIGGER E_D_DEL
INSTEAD OF DELETE ON EMP_DEPT
REFERENCING OLD AS OLDEMP
FOR EACH ROW
DELETE FROM EMP AS E WHERE E.EMPNO = OLDEMP.EMPNO;
Using an INSTEAD OF trigger, each requested modification operation made against the view is replaced by the trigger logic. The trigger performs the insert, update, or delete on behalf of the view. No application changes are required because the code is in the trigger which resides in the database.
If you want to read more about INSTEAD OF triggers, I recommend this quite extensive article (albeit for DB2 LUW) out on the IBM Developer Works web site.