I have taken two tables Car and Registration I want to have One to One mapping between these two tables such that each car can have only One Registration .

so i had made Unique On the fORIGN KEY . Is this way correct to achive One to One on the mapping .

As a compliment to Jeanne's post. 'carName' doesn't even sound unique. If you just came up with this to show your idea, then its okay arguably. Otherwise, introducing a surrogate key, if there are no candidates found, is a good idea. Folks prefer surrogate keys over natural, but again it depends upon case to case. Surrogate key is the only solution, in case you have no natural candidates.