为什么要在数据透视表中将外键定义为主键？

Question

Today I faced something, Someone defined foreign keys as primary keys in Pivot Table! Why is that? Does it effect on performance ? What are the benefits and use cases ? Should we define foreign keys as primary keys in pivot table in order to make them unique ??

Example of my question:

Why should define our many-to-many relationship foreign keys as primary keys ?

Answer 1

This is the proper way to build a junction table, aka bridge table, in a NM relationship.

Rationale:

• each order_id (resp user_id ) should refer to a record in master table orders (resp users ), so you do want a foreign key on each of these columns to enforce data integrity

• each (order_id, user_id) tuple should occur not more than once in the bridge table: this can be enforced by using this columns tuple as a composite primary key, or by setting a unique constraint on it.

Of course, it is also possible to define an indenpendent primary key for the junction table (like an auto-incrementend pk), but that is mostly a matter of taste.