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

SELECT id, 1 AS sort_order
  FROM b
UNION
SELECT id, 2 AS sort_order
  FROM a
MINUS
SELECT id, 2 AS sort_order
  FROM b
ORDER BY 2;