Two options could be:

Code:
SELECT a1,
       a2
FROM   table1 t1
WHERE  NOT EXISTS (SELECT NULL
                   FROM   table2 t2
                   WHERE  t1.a1 = t2.a1
                   AND t1.a2 = t2.a2);


SELECT *
FROM   table1
WHERE  (a1, a2) NOT IN (SELECT a1,
                               a2
                   	FROM   table2
	                WHERE  a1 IS NOT NULL 
                    	AND a2 IS NOT NULL );