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

@Adrien's answer is not working. It gives an ORA-01791.

The correct answer (for the question that is asked) should be:

select id
from 
 (SELECT id, 2 as ordered FROM a -- returns 1,4,2,3
  UNION ALL
  SELECT id, 1 as ordered FROM b -- returns 2,1
  )
group by id
order by min(ordered)

Explanation:

1. The "UNION ALL" is combining the 2 sets. A "UNION" is wastefull because the 2 sets could not be the same, because the ordered field is different.
2. The "group by" is then eliminating duplicates
3. The "order by min (ordered)" is assuring the elements of table b are first

This solves all the cases, even when table b has more or different elements then table a