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| DATE | 2015-02-26 |
| FROM | Ruben Safir
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| SUBJECT | Subject: [LIU Comp Sci] [mrbrklyn@panix.com: (fwd) Re: Role of functional dependencies in
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Date: Thu, 26 Feb 2015 07:10:30 -0500
From: Ruben Safir
To: learn-at-nylxs.com
Subject: [LIU Comp Sci] [mrbrklyn-at-panix.com: (fwd) Re: Role of functional dependencies in
database design]
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----- Forwarded message from Ruben Safir -----
Date: Thu, 26 Feb 2015 07:08:29 -0500 (EST)
From: Ruben Safir
To: mrbrklyn-at-panix.com
Subject: (fwd) Re: Role of functional dependencies in database design
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From: Nicola
Newsgroups: comp.databases.theory
Subject: Re: Role of functional dependencies in database design
Date: Fri, 20 Feb 2015 10:08:54 +0100
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References: <80f34c94-e62e-4d02-8244-7e928acdf476-at-googlegroups.com>
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Xref: panix comp.databases.theory:75565
In article <80f34c94-e62e-4d02-8244-7e928acdf476-at-googlegroups.com>,
compdb-at-hotmail.com wrote:
> > given two lecturers L1 and L2 and a time T
> > when L1 is teaching at time T and L2 is teaching at time T
> > then L1 and L2 are in different rooms
>
> FORALL L1,L2,T,R1,R2
> teaches(L1,T,R1) AND teaches(L2,T,R2) => R1 <> R2
I think that it should be:
FORALL L1,L2,T,R1,R2
L1 <> L2 AND teaches(L1,T,R1) AND teaches(L2,T,R2) => R1 <> R2
> > (this is absolutely off the top of my head: don't take it as a serious
> > attempt).
>
> No FD.
Correct. The FD is implied, though:
Let X be L1 <> L2
Let Y be teaches(L1,T,R1) AND teaches(L2,T,R2)
Let Z be R1 <> R2
Then the sentence above is the universal closure of
X AND Y -> Z,
If my coffee has worked as it should, the latter is equivalent to
(NOT Z) AND Y -> NOT X,
that is,
R1 = R2 AND teaches(L1,T,R1) AND teaches(L2,T,R2) -> L1 = L2,
or, simplifying,
teaches(L1,T,R) AND teaches(L2,T,R) -> L1 = L2.
The idea behind my example is that a hypothetical formalism based on
natural language might allow the user to write specifications that are
not a direct translation of the logical definition of a FD, but from
which the system might *infer* a FD, giving the user more freedom in the
way constraints may be expressed. But again, I haven't really thought
about it too much.
Nicola
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