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

Subquery:

SELECT id, SUM(amount)
FROM ( SELECT * FROM t1
       UNION ALL SELECT * FROM t2
       UNION ALL SELECT * FROM t3
     )
GROUP BY id