Full Join
A full join will give you the union of A and B, i.e. All the rows in A and all the rows in B. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa.
select A.Id, B.Id
from TableA A
FULL JOIN TableB B
on A.Id = B.Id;
The result will be
A.Id | B.Id
-------+-------
1 | null
2 | null
3 | 3
4 | 4
null | 6
null | 5
If you just want rows eith in A or in B, then you have to add a where caluse a.a is null or b.b is null to exclude any common rows.
select A.Id, B.Id
from TableA A
FULL JOIN TableB B
on A.Id = B.Id
where
(A.Id is null) or (B.Id is null);
The result will be
A.Id | B.Id
-------+-------
1 | null
2 | null
null | 6
null | 5