Re: Domain Functional Dependancy?

Kyle Lahnakoski wrote:
>
> I am wondering about two fields where one defines the domain of the
> next. Can this be put in terms of standard functional dependencies?
>
> EG. A=Fruit allows B to be one of (Apple, Orange)
> A=Vegetable allows B to be one of (Carrot, Cabbage)
>
> What if the domains intersect?

Well, even if they don't intersect you still have a problem. Sure, you can add the FD B -> A but that doesn't really capture the complete semantics. For that you would need an extra table Sort(A,B) that records what sort of A every B is, and then add a foreign key constraint that says that (A,B) in the relation refers to (A,B) in the relation Sort. And then it doesn't matter if the domains intersect or not.

-- 
Kind regards,

    Jan Hidders