Problem with trigger for avoiding insert (Oracle SQL) - sql

Question

I'm trying to make a trigger to avoid inserting data in the table for this reason:
If a patient is dead he can't make a visit.
I made a function that return a 1 if the patient is dead and I call it on the trigger. That's the code:
CREATE OR REPLACE FUNCTION check_d (pd INTEGER)
RETURN INTEGER
IS
d Patient.d_date%TYPE;
BEGIN
SELECT P.d_Date INTO d
FROM Patient P
WHERE P.idP=pd;
IF d <> NULL THEN
RETURN 1;
ELSE
RETURN 0;
END IF;
END;
CREATE OR REPLACE TRIGGER bad_insert
BEFORE INSERT ON Visit
FOR EACH ROW
DECLARE
idd INTEGER;
dp Visit.id_PV%TYPE;
BEGIN
SELECT V.id_PV INTO dp
FROM Visit V
WHERE id_PV=:NEW.id_PV;
idd := check_d(dp);
IF idd = 1 THEN
RAISE_APPLICATION_ERROR(-20014,'Error. Patient Deceased.');
END IF;
END;
/
The function should be ok, the problem is in thet trigger.
Visit.id_PV is a Foreign Key to Patient.idP(PRIMARY KEY) anyway. How should I modify the code? Thanks.

Answer 1

Your function is not correct
d <> NULL
Will always return UNKNOWN, instead you have to check for nulls like:
d is not null
Once you've fixed that, you still have to work around the mutating table error you'd receive when you try an insert statement (did you not receive this when you tried it?). You are referencing the same table you are inserting into within your trigger. You can't do that. Instead you should reference the column you want using :new.id_PV .

Answer 2

I think the problem is located here :
IF d <> NULL THEN
Change it to :
IF d IS NOT NULL THEN

Answer 3

In the current case, you'll get no_data_found exception because of the query within the trigger. Moreover, you do not need to return the value for id_PV column that you already have as :NEW.id_PV, and such attempt mostly end with mutating trigger error. So , rearrange your trigger as
CREATE OR REPLACE TRIGGER bad_insert
BEFORE INSERT ON Visit
FOR EACH ROW
BEGIN
IF check_d(:NEW.id_PV) = 1 THEN
RAISE_APPLICATION_ERROR(-20014, 'Error. Patient Deceased.');
END IF;
END;
/
while getting rid of redundant local variables.
The function might also be made brief such as
CREATE OR REPLACE FUNCTION check_d (pd Visit.id_PV%type) RETURN INT IS
val INT;
BEGIN
SELECT NVL(SUM(NVL2(P.d_Date,1,0)),0)
INTO val
FROM Patient P
WHERE P.idP=pd;
RETURN val;
END;
/
Demo