Question

In our DB we have two tables A, B with primary keys A_id and B_id.

Is it a considered a good practice to have B_id as foreign key in table A and A_id as foreign key in table B. This would allow us to have many-to-many relationship in the table.

An alternative would be to have a third bridge table consisting of just two columns A_id and B_id.

Which one do you think is a good practice?

Answer 1

Consider following scenario

TableA  TableB
A       1
B       2

If you want to crosslink this, the least you need to do without creating a third table is duplicating every row in one of the two tables. I doubt you’ll find many DBA’s willing to model their tables like that.

TableA
A, 1  
A, 2   
B, 1
B, 2

TableB
1
2   

A third bridge table really is your only good option here.