How to rewrite this query not to use union clause

Question

How to rewrite this query NOT to use UNION (UNION ALL) clause:

SELECT
  c
FROM a
UNION
SELECT
 c
FROM b

expected result (recordset should be the same):

SELECT
  c
FROM ....

Answer 1

To get the same results as your query above, you can do this:

SELECT COALESCE(a.c, b.c) AS c
FROM a
FULL OUTER JOIN b
    ON b.c = a.c

However, this will give you the same results as a UNION which is not quite the same as a UNION ALL (as duplicates will be removed). To do a UNION all, you'd need to do the same but have the join condition fail:

SELECT COALESCE(a.c, b.c) AS c
FROM a
FULL OUTER JOIN b
    ON 1 = 0

In either case, I'm not sure it would be much faster than doing it using UNION.