Re: ?? Functional Dependency Question ??

tlbaxter99_at_yahoo.com wrote:
> Hi all,
>
> I'm reading Elmasri & Navathe's "Fundamentals of Database Systems, 4th
> ed.". The authors discuss how, given a set if FDs, additional FDs can
> be inferred. The authors provide six "Inference Rules". At one point
> the authors say this:
>
> "Although X->A and X->B implies X->AB by the union rule stated above,
> X->A, and Y->B does *not* imply that XY->AB."
>
> I'm not seeing this. It seems to me that X->A, and Y->B *DOES* imply
> that XY->AB.
>
> I'm sure I'm wrong but I'm not seeing it. Can someone explain?
>
> Thanks

Here's what I get using Armstrong's Axioms:

i) X -> A (given)
ii) XY -> AY (augment i) with Y)

iii) Y -> B (given)
iv) AY -> AB (augment iii) with A )

v) XY -> AB (ii), iv), transitivity)

Maybe the book has a typo? If so, I wonder what they meant to say? Received on Sat Oct 18 2008 - 17:36:38 CEST