Question

I have three tables, tableA (id, A), tableB (id,B) and tableC (id,C). id is unique and primary key. Now I want to run a query on these three tables to find out sum of values of A,B and C for every id. (i.e. if id 1 is present in tableA but not in tableB then value B should be considered as 0 for id 1).example: tableA:

id  A
1   5
2   6
3   2
5   7

tableB:

id  B
2   5
3   8
4   1

tableC:

id  C
5   2

the output should be:

id  Sum
1   (5 + 0 + 0 =)5
2   (6 + 5 + 0 =)11
3   (2 + 8 + 0 =)10
4   (0 + 1 + 0 =)1
5   (7 + 0 + 2 =)9

Answer 1

First get a distinct list ( UNION ) of the IDs so that you include all, then LEFT JOIN to add the values together.

Something like

SELECT  IDs.ID,
         IFNULL(tableA.A,0) + IFNULL(tableB.B,0) + IFNULL(tableC.C,0) SumVal
FROM    (
            SELECT  ID
            FROM    tableA
            UNION
            SELECT  ID
            FROM    tableB
            UNION
            SELECT  ID
            FROM    tableC
        ) IDs LEFT JOIN
        tableA ON IDs.ID = tableA.ID LEFT JOIN
        tableB ON IDs.ID = tableB.ID LEFT JOIN
        tableC ON IDs.ID = tableC.ID