a simple way to sum a result from UNION in MySql

Question

I have a union of three tables (t1,t2,t3). Each rerun exactly the same number of records, first column is id, second amount:

1  10
2  20
3  20

1  30
2  3         


        

Answer 1

SELECT id, SUM(amount) FROM
(
    SELECT id, SUM(amount) AS `amount` FROM t1 GROUP BY id
  UNION ALL
    SELECT id, SUM(amount) AS `amount` FROM t2 GROUP BY id
) `x`
GROUP BY `id`

I groupped each table and unioned because i think it might be faster, but you should try both solutions.