Question

I’m creating a mock up database for the first time. I have created the Relational Model which consists of a many to many relationship. In the Relational Model it has a separate linking/joining table. When creating the database do I need to create this linking table as a separate table also? Or can I just put each foreign keys in the many to many tables?

If I need a separate table how do I link these tables together via syntax?

Thanks

Answer 1

We can only build foreign keys in a one-to-many fashion. So you need this intersection table. It is the sort of additional construct we introduce when transforming a logical data model into a physical one.

The intersection table often has just two columns, the referencing keys of the two tables you want to link in M:N fashion (there may also be some metadata columns to hold properties of the link). It usually has a compound primary key on the two columns, to avoid redundancy. It has a foreign key on each of the referenced tables, which must have defined primary keys on the referenced columns.

The syntax is pretty obvious; this sample builds two master tables and an intersection defining just the keys.

create table m1 (
   id number not null
   , constraint m1_pk primary key (id) );


create table m2 (
   id number not null
   , constraint m2_pk primary key (id) );


create table intersection_t (
   m1_id number not null
   , m2_id number not null
   , constraint int_pk primary key (m1_id, m2_id)
   , constraint int_m1_fk foreign key (m1_id)
         references m1 (id)
   , constraint int_m2_fk foreign key (m2_id)
         references m2 (id)
 );