Enforcing “zero-or-one to one” relationship on SQL database?

Question

I have a Post entity and a FbPost entity.

Post.FbPost is either null or a FbPost, and no two Post en

Answer 1

Create a unique filtered index:

CREATE UNIQUE INDEX Post_Unq_FbPost ON dbo.Post(FbPost) WHERE FbPost IS NOT NULL;

Also create a foreign key, of course.