Creating a pseudo foreign key trigger

Question

hi
i want to create a trigger that will act like a foreign key and check whether the value being entered already exists in two separate tables. I want it to not allow entry if the id already exists in: either none of the tables; or both the tables.
I have tried to create the following trigger, it compiles, but doesnt do the job :(
create or replace trigger trig_test1
before insert on TEST3
referencing old as old new as new
for each row
DECLARE
cursor cur_check_test1 is
select PKONE
from TEST1, TEST3
where PKONE = TEST3.PKTHREE;
CURSOR CUR_CHECK_TEST2 IS
SELECT PKTWO
FROM TEST2, TEST3
WHERE PKTWO = TEST3.PKTHREE;
v_key_conflict EXCEPTION;
v_test1_temp varchar2(1);
v_test2_temp varchar2(1);
BEGIN
OPEN cur_check_test1;
FETCH cur_check_test1 into v_test1_temp;
if (((cur_check_test1%FOUND) AND (cur_check_test2%found)) or ((cur_check_test1%notfound) and (cur_check_test2%notfound)))
then close cur_check_test1;
close cur_check_test2;
RAISE v_key_conflict;
else
close cur_check_test1;
close cur_check_test2;
end if;
EXCEPTION
WHEN v_key_conflict then
raise_application_error(-20111, 'parent key constraint violated');
end;
Can someone please help me out. cheers

Answer 1

What business requirement are you trying to meet?
It's going to be difficult to correctly implement this kind of constraint using triggers, because:
1. You will need to handle updates on TEST3, not just inserts.
2. You will need to handle inserts, updates, and deletes on both TEST1 and TEST2.
3. You will need some sort of locking mechanism to prevent concurrent transactions from violating the constraint (because they can't see the changes that other transactions have made but have not yet committed).
In other words, it might be on the order 10 times more complicated to implement this kind of constraint using triggers than you originally thought. Before pursuing the trigger approach further, it might be worth looking into some alternatives. Is the benefit of implementing the constraint worth the effort? Are there some "creative" ways you can use declarative (built-in) constraints instead? Would restructuring your tables help?