?? Functional Dependency Question ??
From: <tlbaxter99_at_yahoo.com>
Date: Fri, 17 Oct 2008 22:33:19 -0700 (PDT)
Message-ID: <a66c0136-9b90-4506-ac35-1d2c67d980be_at_h2g2000hsg.googlegroups.com>
Hi all,
Date: Fri, 17 Oct 2008 22:33:19 -0700 (PDT)
Message-ID: <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