How to delete/update records inside trigger based on the updated/deleted row? - sql

Question

I have a table with the following format
id | name | supervisor_id
I made a "BEFORE INSERT" trigger that checks if the supervisor_id exists in the id column and if not, then assign a null value to the supervisor_id.
I am trying to write two more triggers. One that checks if the supervisor_id exists in the id column before each update of the supervisor_id, and one that sets the supervisor_id to NULL for each employee if his supervisor is deleted.
This is my code, of course it's not working, help please.
CREATE OR REPLACE TRIGGER EAP_users_TRG3
AFTER DELETE
ON EAP_users
FOR EACH ROW
DECLARE
d NUMBER;
BEGIN
SELECT id INTO d FROM EAP_users WHERE id = :OLD.id;
UPDATE EAP_users SET supervisor = NULL WHERE supervisor = d;
END;
/
This is the "working" trigger:
CREATE OR REPLACE TRIGGER EAP_users_TRG1
BEFORE INSERT
ON EAP_users
FOR EACH ROW
DECLARE
supervisor EAP_users.supervisor%TYPE;
CURSOR supervisor_CUR IS SELECT idFROM EAP_users;
b BOOLEAN := FALSE;
BEGIN
IF ( :NEW.supervisor IS NOT NULL ) THEN
FOR s IN supervisor_CUR LOOP
IF ( :NEW.supervisor = s.id ) THEN
b := TRUE;
END IF;
END LOOP;
IF (b = FALSE) THEN
:NEW.supervisor := NULL;
END IF;
END IF;
END;
/

Answer 1

According to the definition of your problem, you are trying to enforce referential integrity of your data. In that case, a trigger is probably not the right tool. To quote Oracle's documentation:
You can use both triggers and integrity constraints to define and enforce any type of integrity rule. However, Oracle strongly recommends that you use triggers to constrain data input only in the following situations:
[...]
When a required referential integrity rule cannot be enforced using the following integrity constraints:
NOT NULL, UNIQUE
PRIMARY KEY
FOREIGN KEY
CHECK
DELETE CASCADE
DELETE SET NULL
In that particular case you should use FOREIGN KEY constraint using the DELETE SET NULL modifier. Assuming you have an index on id, all you need is:
ALTER TABLE EAP_users
ADD CONSTRAINT EAP_users_supervisor_cst
FOREIGN KEY (supervisor_id)
REFERENCES EAP_users(id)
ON DELETE SET NULL;
This simple referential integrity constraint will perform probably better the same things as your 3 triggers -- namely:
prevent insert/update with a non existing (non-NULL) supervisor_id
set all supervisor_id to NULL when you delete the supervisor
See http://sqlfiddle.com/#!4/1f8fb/1 for a live example.