(My)SQL full join with three tables

Question

I have tree tables

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

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

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

Can anybody p

Answer 1

As far as I know there is no full outer join in MySql. So, 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

Answer 2

select coalesce(a.Id, b.Id, c.Id) ID, 
       case when a.A > 0 then a.A else 0 end, 
       case when b.B > 0 then b.B else 0 end,
       case when c.C > 0 then c.C else 0 end,
       ifnull(a.A, 0) + ifnull(b2.B, 0) - ifnull(c.C, 0) R
 from tableA a
 right outer join tableB b on a.id = b.id
 right outer join tableC c on b.id = c.id;

Answer 3

Try this

  select t1.ID, t1.A,t2.B,t3.C,t1.A+t2.B-t3.C as R
  from tableA t1
  full outer join tableB t2 on t1.id =t2.id
  full outer join tableC t3 on t1.id =t3.id