SQL: how to use UNION and order by a specific select?

Question

I have two selects:

SELECT id FROM a -- returns 1,4,2,3
UNION
SELECT id FROM b -- returns 2,1

I\'m receiving correct num of rows, like:

Answer 1

@Adrian's answer is perfectly suitable, I just wanted to share another way of achieving the same result:

select nvl(a.id, b.id)
from a full outer join b on a.id = b.id
order by b.id;