Question

I am having a problem with MySQL joins.

Table_A:

 A_id  Cost1  A1_id  Cost2  1     500     0     200  1     100     1     100  1     50      2      60  1     10      3      50  2      5      0      10 

Table_B (Refers B_id: from Table_A A1_id):

 B_id  FName  LName  1     X      A  2     Y      B  3     Z      C 

Table_C (Refers C_id: from Table_A A_id):

C_id  Towns   1    Atlanta  2    NewYork 

I need to combine all three tables, like the following output:

• I extract the Towns that match (Table_A.A_id=Table_C.C_id).
• I extract the Fname,Lname that match (table_A.A1_id=Table_b.b_id).
• I need to skip the Towns if A1_id != 0.
• I need to skip the Fname,Lname if A1_id == 0.

The remaining data may either be a value or null, which I specify as ‘#’. What would be an efficient MySQL query for the given scenario?

Output:

A_id  Cost1  A1_id cost2 Fname Lname Towns 1     500     0    200    #     #    Atlanta 1     100     1    100    X     A     # 1      50     2     60    Y     B     # 1      10     3     50    Z     C     # 

Answer 1

This looks like a UNION of two distinct queries to me. I’m going to assume that the ID columns never contain negative values.

SELECT A.A_id, A.Cost1, A.A1_id, A.Cost2, B.Fname, B.Lname, C.Town     FROM Table_A AS A               INNER JOIN Table_B AS B ON A.A1_id = B.B_id          LEFT OUTER JOIN Table_C AS C ON A.A_id  = C.C_id     WHERE A.A1_id != 0       AND C.C_id < 0 UNION SELECT A.A_id, A.Cost1, A.A1_id, A.Cost2, B.Fname, B.Lname, C.Town     FROM Table_A AS A          LEFT OUTER JOIN Table_B AS B ON A.A1_id = B.B_id               INNER JOIN Table_C AS C ON A.A_id  = C.C_id     WHERE A.A1_id = 0       AND B.B_id < 0; 

I’m not completely confident about that formulation. There’s a chance that the conditions on B.B_id < 0 and C.C_id < 0 need to be associated with the corresponding ON clauses.

There’s also a decent chance that using two left outer joins in a single SELECT with appropriate OR’d filters would achieve the correct result.