Every trigger is associated with an event. Triggers are
activated when their corresponding event occurs in the database. This
trigger event occurs when the specified action, either an UPDATE,
INSERT, or DELETE statement (including those caused by actions of
referential constraints), is performed on the target table.
About this task
For example:
CREATE TRIGGER NEW_HIRE
AFTER INSERT ON EMPLOYEE
FOR EACH ROW
UPDATE COMPANY_STATS SET NBEMP = NBEMP + 1
The preceding
statement defines the trigger new_hire, which activates when you perform
an insert operation on table employee.
You associate every trigger
event, and consequently every trigger, with exactly one target table
and exactly one modify operation. The modify operations are:
- Insert operation
- An insert operation can only be caused by an INSERT statement
or the insert operation of a MERGE statement. Therefore, triggers
are not activated when data is loaded using utilities that do not
use INSERT, such as the LOAD command.
- Delete operation
- A delete operation can be caused by a DELETE statement, or the
delete operation of a MERGE statement, or as a result of a referential
constraint rule of ON DELETE CASCADE.
- Update operation
- An update operation can be caused by an UPDATE statement, or the
update operation of a MERGE statement, or as a result of a referential
constraint rule of ON DELETE SET NULL.
If the trigger event is an update operation,
the event can be associated with specific columns of the target table.
In this case, the trigger is only activated if the update operation
attempts to update any of the specified columns. This provides a further
refinement of the event that activates the trigger.
For example,
the following trigger, REORDER, activates only if you perform an update
operation on the columns ON_HAND or MAX_STOCKED, of the table PARTS:
CREATE TRIGGER REORDER
AFTER UPDATE OF ON_HAND, MAX_STOCKED ON PARTS
REFERENCING NEW AS N_ROW
FOR EACH ROW
WHEN (N_ROW.ON_HAND < 0.10 * N_ROW.MAX_STOCKED)
BEGIN ATOMIC
VALUES(ISSUE_SHIP_REQUEST(N_ROW.MAX_STOCKED -
N_ROW.ON_HAND,
N_ROW.PARTNO));
END
When a trigger is activated, it runs according
to its level of granularity as follows:
- FOR EACH ROW
- It runs as many times as the number of rows in the set of affected
rows. If you need to refer to the specific rows affected by the triggered
action, use FOR EACH ROW granularity. An example of this is the comparison
of the new and old values of an updated row in an AFTER UPDATE trigger.
- FOR EACH STATEMENT
- It runs once for the entire trigger event.
If the set of affected rows is empty (that is, in the
case of a searched UPDATE or DELETE in which the WHERE clause did
not qualify any rows), a FOR EACH ROW trigger does not run. But a
FOR EACH STATEMENT trigger still runs once.
For example, keeping
a count of number of employees can be done using FOR EACH ROW.
CREATE TRIGGER NEW_HIRED
AFTER INSERT ON EMPLOYEE
FOR EACH ROW
UPDATE COMPANY_STATS SET NBEMP = NBEMP + 1
You
can achieve the same affect with one update by using a granularity
of FOR EACH STATEMENT.
CREATE TRIGGER NEW_HIRED
AFTER INSERT ON EMPLOYEE
REFERENCING NEW_TABLE AS NEWEMPS
FOR EACH STATEMENT
UPDATE COMPANY_STATS
SET NBEMP = NBEMP + (SELECT COUNT(*) FROM NEWEMPS)
Note: - A granularity of FOR EACH STATEMENT is not supported for BEFORE
triggers.
- The maximum nesting level for triggers is 16. That is, the maximum
number of cascading trigger activations is 16. A trigger activation
refers to the activation of a trigger upon a triggering event, such
as insert, update, or delete of data in a column of a table, or generally
to a table.