Re: Enforcing functional dependecy constraints
From: David Cressey <david.cressey_at_earthlink.net>
Date: Fri, 09 Dec 2005 12:40:31 GMT
Message-ID: <3Xemf.1296$nm.109_at_newsread2.news.atl.earthlink.net>
> Thanks, David C. You're right and I'm wrong again. Just made another
> example for myself and using my earlier substitutes U (all-key) allows a
> teacher to use several books.
> But the FD AB->C says that a teacher can't use the same book for two
> different courses. Since U is many-to-one (ie. many C to one B), the
> join could result in rows where many C's show up for the same B and thus
> the same (A,B), allowing a teacher to use the same book for two
> different courses.
> Thinking about this, I wonder if it is because C, being the the
> determinant of the stricter FD, must be in both projections (whereas
> that's not so with U(A,B) and T(C,B).
> I guess I'll have to go study the theory again. It's not a waste of my
> time, but sorry for wasting everybody else's!
Date: Fri, 09 Dec 2005 12:40:31 GMT
Message-ID: <3Xemf.1296$nm.109_at_newsread2.news.atl.earthlink.net>
"paul c" <toledobythesea_at_oohay.ac> wrote in message
news:eY6mf.71805$ki.39213_at_pd7tw2no...
> David Cressey wrote:
> > "paul c" <toledobythesea_at_oohay.ac> wrote in message
> > news:WwKlf.63515$ki.5786_at_pd7tw2no...
> >
> >>paul c wrote:
> >>
> >>>...
> >>>
> >>>I agree with David but also I guess that the above comes down to saying
> >>>that BCNF is possible either with S(A,C) and T(C,B) or with U(A,B) and
> >>>T(C,B), assuming that C is a key.
> >>>
> >>
> >>Oops, meant to say "assuming C is a key of T".
> >>
> >>
> >>p
> >
> >
> > I don't see how to reconstruct R (A,B,C) from U(A,B) and T(C,B)
> >
> >
> >
> >
>
> Thanks, David C. You're right and I'm wrong again. Just made another
> example for myself and using my earlier substitutes U (all-key) allows a
> teacher to use several books.
> >
> But the FD AB->C says that a teacher can't use the same book for two
> different courses. Since U is many-to-one (ie. many C to one B), the
> join could result in rows where many C's show up for the same B and thus
> the same (A,B), allowing a teacher to use the same book for two
> different courses.
> >
> Thinking about this, I wonder if it is because C, being the the
> determinant of the stricter FD, must be in both projections (whereas
> that's not so with U(A,B) and T(C,B).
> >
> I guess I'll have to go study the theory again. It's not a waste of my
> time, but sorry for wasting everybody else's!