(My)SQL full join with three tables

I have three tables

ID    A
-----------
1     10

ID    B
-----------
1     20
2     30



ID    C
-----------
2     40
3     50

The result after merge

ID     A      B      C      R (A + B - C)
-----------------------------------



1     10     20      0     30
2      0     30     40    -10
3      0      0     50    -50

To do what you require you should get distinct IDs in derived table and left join original tables:

select ids.id,
       ifnull(table1.A, 0) A,
       ifnull(table2.B, 0) B,
       ifnull(table3.C, 0) C,
       ifnull(table1.A, 0) + ifnull(table2.B, 0) - ifnull(table3.C, 0) R
  from 
  (
    select id
      from table1
    union
    select id
      from table2
    union
    select id
      from table3
  ) ids
  left join table1
    on ids.id = table1.id
  left join table2
    on ids.id = table2.id
  left join table3
    on ids.id = table3.id

Nguồn: http://stackoverflow.com/