Consulta SQL para encontrar linhas ausentes entre duas tabelas relacionadas

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)

SELECT A.ABC_ID, A.VAL FROM A WHERE VAL NOT IN 
    (SELECT VAL FROM B WHERE B.ABC_ID = A.ABC_ID)

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