How to enforce the uniqueness of a relationship?

Hi,

We went through an HR-Application workshop/exercise and watch several videos about VB.

In that workshop we worked with three Business objects: (Location, Department and Employee).

I wonder, how can we enforce the uniqueness of a relationship?

For example, how can we implement that a department has one to one relationship to a location?

The same applies to the employee-department relationship, an employee can only be a member in one department.

Regards

Thabet