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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 07:11:01 -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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From: Ruben Safir
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Subject: (fwd) Re: Role of functional dependencies in database design
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Subject: Re: Role of functional dependencies in database design
From: Erwin
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Xref: panix comp.databases.theory:75526
Op donderdag 12 februari 2015 19:58:19 UTC+1 schreef Erwin:
> Op donderdag 12 februari 2015 17:02:19 UTC+1 schreef Nicola:
> > Well no, like this certainly no :) Maybe something along these lines=20
> > (paraphrasing the given-when-then constructs existing in some testing=
=20
> > tools):
> >=20
> > 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
>=20
> ... convoluted ...
I now see why I feel so uncomfortable with this formulation, and why it doe=
sn't reveal the real FD right away.
First, yet another alternative formulation of the same.
Given two lecturers L1 and L2 and a room R
when L1 is teaching in room R and L2 is teaching in room R
then this is at different times.
The FD is {room, time} on the LHS. room and time are of equal "importance"=
here, they play "similar roles". But now look at the word usage for "room=
" and "time". Your original speaks of "different rooms" but _NOT_ of diffe=
rent times (quite the contrary) !
The _formulation_ in natural language "breaks" a certain kind of "symmetry"=
that is indeed present in the math formulation. That makes the discovery/=
analysis process harder !
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