Trigger that checks if the same tuple exists in two different tables - sql

Question

I wrote the following code:
CREATE OR REPLACE TRIGGER CHECK_tuple
BEFORE INSERT ON tableB
FOR EACH ROW
DECLARE IS_JOIN BOOLEAN:=FALSE
BEGIN
SELECT tableB.column1, tableB.column2,
CASE
WHEN IS_JOIN:= FALSE THEN raise_application_error(-20101, 'ERROR.');
ELSE IS_JOIN:= TRUE
END AS CHCK_JOIN
FROM tableB
JOIN tableA
ON tableB.column1=tableA.column1 AND tableB.column2=tableA.column2;
END;
I have to check if a tuple (t1) exits in table A (with "tuple", i mean the entire row of the table with multiple columns). If exists, it has to match with t2 in table B. Before one inserts tuple t2 in table B, the trigger must activate. If t1 doesn't match with t2, the flag IS_JOIN will remain FALSE and Oracle SQL will give an error. Else, if t1 is equal to t2, IS_JOIN will be TRUE and no action will be take. I want this "check" to take place for each row that one will insert in table B. Is this the proper way to do it? If the task isn't clear, please ask for further info.

Answer 1

The proper way to do something like this:
CREATE OR REPLACE TRIGGER test_test_CHECK_tuple
BEFORE INSERT ON tableB
FOR EACH ROW
declare
v_cnt number(10);
BEGIN
SELECT count(*)
into v_cnt
FROM tableA
where column1=:new.column1
and column2=:new.column2;
if v_cnt= 0 then
raise_application_error(-20101, 'ERROR.');
end if;
END;
:new means, that are the values, you want to insert. There is no other way to use that values.
btw. that is not really how a foreign key works, since a foreign key is assigned to a primary key or unique key