Many to Many Relation Design - Intersection Table Design

Question

I\'m wondering what a better design is for the intersection table for a many-to-many relationship.

The two approaches I am considering are:

CREATE TA         


        

Answer 1

The second is better. It constrains the junction box (intersection box) to not having more than one appearance of the same pair. If there are no other columns in the junction box, you won't be doing lookups of this table anyway, except by way of the two foreign keys.