SELECT A.ABC_ID, A.VAL FROM A WHERE NOT EXISTS
(SELECT * FROM B WHERE B.ABC_ID = A.ABC_ID AND B.VAL = A.VAL)
of
SELECT A.ABC_ID, A.VAL FROM A WHERE VAL NOT IN
(SELECT VAL FROM B WHERE B.ABC_ID = A.ABC_ID)
of
SELECT A.ABC_ID, A.VAL LEFT OUTER JOIN B
ON A.ABC_ID = B.ABC_ID AND A.VAL = B.VAL FROM A WHERE B.VAL IS NULL
Houd er rekening mee dat deze zoekopdrachten helemaal niet vereisen dat ABC_ID in tabel B staat. Ik denk dat dat doet wat je wilt.