Subquery in IF statement in trigger, without using foreign keys

Question

Hello,
I'm investigating ways of writing a subquery in an IF statement, which is placed inside a trigger.
I wanna write smth like IF (:new.jazz not in (select goldies from T where ... )) etc. I don't know whether the fact that the IF is in a trigger adds some additional restrictions. (Does it?)
So far I found the solution described here: SubQuery Comparison in If Statement which I find a bit tacky, I could have the 'cooleststarinthegalaxy' instead of 1 and seems you need to do extra light, but still extra lifting.
I also read about the possibility of using MERGE, which I'm currently researching.
Is there any other way?
Thanks
Edited by: BluShadow on 14-Nov-2012 13:37
fixed link
Edite by me: the question is how (if possible) to do this without a foreign key.
Edited by: questioningq12 on Nov 14, 2012 6:11 AM
Edited by: questioningq12 on Nov 14, 2012 6:13 AM

Answer 1

Hi,
questioningq12 wrote:
Say I have tables A(namea varchar(10)), B(nameb varchar(10)), and B contains tuples ('1stname','2ndname').
I wrote a trigger before insertion, for each row, on table A. For a tuple t to be inserted, it should check whether t.namea is in the set of values nameb from B.
E.g., INSERT INTO A VALUES('1stname') should work. But INSERT INTO A VALUES ('3rdname') should fail. You can use a foreign key constraint for that.
If the tables already exist, and b.nameb is declared as UNIQUE (or PRIMARY KEY), then you can say:
ALTER TABLE  a
    ADD CONSTRAINT     a_namea_fk
    FOREIGN KEY  (namea)
    REFERENCES b (nameb)
;If you had a situation where you really needed to query a table in PL/SQL, and you weren't sure if the query would find anything, you could put the query in its own BEGIN ... EXCEPTION block, and test for NO_DATA_FOUND.
If you're just checking to see if a row exists or not, you can always write a query that is guaranteed to return exactly 1 row, like this:
BEGIN
    SELECT  COUNT (*)
    INTO    x
    FROM    b
    WHERE   nameb = :NEW.namea
    AND         ROWNUM  = 1;
    IF x = 0
    THEN  
        ...        -- print msgs, raise exceptions etc
    END IF;
END;Edited by: Frank Kulash on Nov 14, 2012 9:22 AM
Added example