create table tst1 (c1 number,c2 number,c3 number);
create table tst2 (c1 number,c2 number);
insert into tst1 values (1,2,null);
insert into tst1 values (3,4,null);
insert into tst1 values (5,6,null);
insert into tst1 values (7,8,null);
insert into tst2 values (1,2);
insert into tst2 values (5,6);
commit;
-------
update tst1 t
   set t.c3 = 1
 where exists (select 1
          from tst2 t2
         where t2.c1 = t.c1
           and t2.c2 = t.c2);
commit;
select * from tst1;
--------
update tst1 t
   set t.c3 = 2
 where (t.c1, t.c2) in (select t2.c1, t2.c2 from tst2 t2);
commit;
select * from tst1;