Question

I would like to establish a many-to-many relationship with a constraint that only one or no entity from each side of the relationship can be linked at any one time.

A good analogy to the problem is cars and parking garage spaces. There are many cars and many spaces. A space can contain one car or be empty; a car can only be in one space at a time, or no space (not parked).

We have a Cars table and a Spaces table (and possibly a linking table). Each row in the cars table represents a unique instance of a car (with license, owner, model, etc.) and each row in the Spaces table represents a unique parking space (with address of garage floor level, row and number). What is the best way to link these tables in the database and enforce the constraint describe above?

(I am using C#, NHibernate and Oracle.)

Answer 1

If you’re not worried about history – ie only worried about “now”, do this:

create table parking (
  car_id references car,
  space_id references space,
  unique car_id,
  unique space_id
);

By making both car and space references unique, you restrict each side to a maximum of one link – ie a car can be parked in at most one space, and a space can has at most one car parked in it.