AFTER SQL triggers

The WHEN condition can be used in an SQL trigger to specify a condition. If the condition evaluates to true, the SQL statements in the SQL trigger routine body are run. If the condition evaluates to false, the SQL statements in the SQL trigger routine body are not run, and control is returned to the database system.

In the following example, a query is evaluated to determine if the statements in the trigger routine body should be run when the trigger is activated.

CREATE TABLE TodaysRecords(TodaysMaxBarometricPressure FLOAT, 
  TodaysMinBarometricPressure FLOAT)

CREATE TABLE OurCitysRecords(RecordMaxBarometricPressure FLOAT, 
  RecordMinBarometricPressure FLOAT)

CREATE TRIGGER UpdateMaxPressureTrigger 
AFTER UPDATE OF TodaysMaxBarometricPressure ON TodaysRecords
REFERENCING NEW AS new_row
FOR EACH ROW MODE DB2ROW
WHEN (new_row.TodaysMaxBarometricPressure>
     (SELECT MAX(RecordMaxBarometricPressure) FROM
     OurCitysRecords))
  UPDATE OurCitysRecords
         SET RecordMaxBarometricPressure =
             new_row.TodaysMaxBarometricPressure

CREATE TRIGGER UpdateMinPressureTrigger
AFTER UPDATE OF TodaysMinBarometricPressure
ON TodaysRecords
REFERENCING NEW AS new_row
FOR EACH ROW MODE DB2ROW
WHEN(new_row.TodaysMinBarometricPressure<
     (SELECT MIN(RecordMinBarometricPressure) FROM
     OurCitysRecords))
  UPDATE OurCitysRecords
         SET RecordMinBarometricPressure =
             new_row.TodaysMinBarometricPressure

First the current values are initialized for the tables.

INSERT INTO TodaysRecords VALUES(0.0,0.0)
INSERT INTO OurCitysRecords VALUES(0.0,0.0)

For the SQL update statement below, the RecordMaxBarometricPressure in OurCitysRecords is updated by the UpdateMaxPressureTrigger.

UPDATE TodaysRecords SET TodaysMaxBarometricPressure = 29.95

But tomorrow, if the TodaysMaxBarometricPressure is only 29.91, then the RecordMaxBarometricPressure is not updated.

UPDATE TodaysRecords SET TodaysMaxBarometricPressure = 29.91

SQL allows the definition of multiple triggers for a single triggering action. In the previous example, there are two AFTER UPDATE triggers: UpdateMaxPressureTrigger and UpdateMinPressureTrigger. These triggers are activated only when specific columns of the table TodaysRecords are updated.

AFTER triggers may modify tables. In the example above, an UPDATE operation is applied to a second table. Note that recursive insert and update operations should be avoided. The database management system terminates the operation if the maximum trigger nesting level is reached. You can avoid recursion by adding conditional logic so that the insert or update operation is exited before the maximum nesting level is reached. The same situation needs to be avoided in a network of triggers that recursively cascade through the network of triggers.