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,amount from table_1 union all
    select id,amount from table_2 union all
    select id,amount from table_3
) x group by id