Re: ?? Functional Dependency Question ??

From: Walter Mitty <wamitty_at_verizon.net>
Date: Sat, 18 Oct 2008 13:06:30 GMT
Message-ID: <qblKk.1902$Rx2.1675_at_nwrddc01.gnilink.net>


<tlbaxter99_at_yahoo.com> wrote in message news:a66c0136-9b90-4506-ac35-1d2c67d980be_at_h2g2000hsg.googlegroups.com...
> 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

If I understand it right, XY does not imply X and Y. Received on Sat Oct 18 2008 - 15:06:30 CEST

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