Foreig Keys to different tables on the same field

Question

Hi, respected Oracle Techs.:
My question is:
Is there a way to reference a field by Foreign Keys to two or more tables' keys in an mutually exclusive way? For example,
TABA.FIELDA references to TABB.FIELDA and exclusively,
TABA.FIELDA references to TABC.FIELDA, it means:
if child TABA.FIELDA does not exists in parent TABB.FIELDA it MUST exists in parent TABC.FIELDA and viceversa.
'One child looking for his existing parent...'
Thanks in advance.
LEONARDO.

Answer 1

Although I agree that combining tableB and tableC is probably the best way to go, if there is some really good reason that this cannot happen, you can use a trigger on tableA to enforce this rule. Something like:
CREATE OR REPLACE TRIGGER tableA_ref
BEFORE INSERT OR UPDATE ON tableA
FOR EACH ROW
DECLARE
l_isfound NUMBER(1) := 0;
BEGIN
   BEGIN
      SELECT 1 INTO l_isfound
      FROM tableB
      WHERE fielda = :new.fielda;
   EXCEPTION WHEN NO_DATA_FOUND THEN
      BEGIN
         SELECT 1 INTO l_isfound
         FROM tableC
         WHERE fielda = :new.fielda;
      EXCEPTION WHEN NO_DATA_FOUND THEN
         NULL;
      END;
   END;
   IF l_isfound <> 1 THEN
      RAISE_APPLICATION_ERROR (-20001,'Referential Integrity Check Failed');
   END IF;
END;The example assumes that fields unique in both tableB and tableC.
TTFN
John