create table aa(a_col int, b_col int);
create table bb(a_col int, b_col int);

insert into aa values(1,20);
insert into aa values(2,30);
insert into aa values(3,40);

insert into bb values(1,10);
insert into bb values(2,15);
insert into bb values(4,28);

select
   case when aa.a_col is null then bb.a_col else aa.a_col end,
   isnull(aa.b_col, 0),
   isnull(bb.b_col, 0)
from
 aa full outer join bb on(aa.a_col = bb.a_col)
order by
   1

이러면 나오네요...