merge /*+ parallel (emp,6) */ 
     into tabA a
    using ( SELECT c.prod_id,
                   c.as1,
                   c.val_amount 
              from tabB b,   
                   tabC c 
            where (b.date < sysdate
                   and b.b_no = c.c_no
                   and b.dimd = c.dimd
          )  YY 
    on (a.prod_id = YY.prod_id )
      when matched then 
        UPDATE set a.asl = YY.as1
                   a.aq1 = a.del * YY.val_amount;