Re: Role of functional dependencies in database design

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
> > (paraphrasing the given-when-then constructs existing in some testing
> > tools):
> >
> > 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
>
> ... convoluted ...

I now see why I feel so uncomfortable with this formulation, and why it doesn'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 different 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 ! Received on Thu Feb 12 2015 - 20:39:40 CET