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)
eller
SELECT A.ABC_ID, A.VAL FROM A WHERE VAL NOT IN
(SELECT VAL FROM B WHERE B.ABC_ID = A.ABC_ID)
eller
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
Bemærk venligst, at disse forespørgsler overhovedet ikke kræver, at ABC_ID er i tabel B. Jeg tror, det gør, hvad du vil.