Re: functional dependencies

Rohan Hathiwala wrote:

> Larry this is the definition that I gave
>
> "In a relation R if we have 2 attributes A and B and suppose an
> FD exists i.e A -> B then for any two tuples t1 and t2 in
> R if t1[A] = t2[A] the we must have t1[B] = t2[B]"
>
> and this is what you gave
>
> "If there exists a tuple t1 in R with attributes A and B,
> and a functional dependency exists A-->B, then it is not possible
> for tuple t2 to exist in R where t1[A] = t2[A] and t1[B] != t2[B]"
>
> If you look at it carefully our definitions are one and the same.

No, they are distinctly different. Yours starts with the premise of having two tuples where A is the determinant and t1[A] = t2[A], which falls apart when A is chosen as a candidate key. It's a subtle wording distinction, but the distinction is there.

Larry Coon
University of California
larry_at_assist.org
and lmcoon_at_home.com Received on Sat Jun 14 2003 - 20:03:01 CEST