Question

I have a three tables, A, B and C. A has many B, and B has many C. What is the best way design primary keys and relationship among these two. Sorry to ask a designing problem, but I don’t know whether to use

1. one extra table to map A(pkey) – B(pkey) – C(pkey) OR
2. two exta tables to map A-B and B-C separately OR
3. use foreign key relationships without using any extra tables (if so please tell me how the key columns should come)

Thank you very much.

Answer 1

Option (3) sounds like the most natural:

A: { id, *, *, ... };

B: { id, a_id references(A.id), *, *, ... }

C: { id, b_id references(B.id), *, *, ... }

To query:

SELECT ... FROM A JOIN B ON (A.id = B.a_id) JOIN C ON (B.id = C.b_id);

Always make your database model the logical relationships in your data, not the other way round!