Re: ?? Functional Dependency Question ??
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sat, 18 Oct 2008 12:18:16 -0300
Message-ID: <48f9fe48$0$5500$9a566e8b_at_news.aliant.net>
Date: Sat, 18 Oct 2008 12:18:16 -0300
Message-ID: <48f9fe48$0$5500$9a566e8b_at_news.aliant.net>
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