Re: The "standard" way to get to 3NF
From: Jonathan Leffler <jleffler_at_earthlink.net>
Date: Sat, 10 Apr 2004 04:31:06 GMT
Message-ID: <eMKdc.3329$k05.841_at_newsread2.news.pas.earthlink.net>
>
> Did anyone notice that this algorithm is actually not correct? Take the
> following example of a relation R(A,B,C,D,E) with the set of FDs:
>
> { AB->C, AB->D, BC->D }
Date: Sat, 10 Apr 2004 04:31:06 GMT
Message-ID: <eMKdc.3329$k05.841_at_newsread2.news.pas.earthlink.net>
Jan Hidders wrote:
> Jan Hidders wrote:
>> [...] The usual algorithm that gets you to 3NF in one step (the one >> using the minimal cover) splits as little as possible. See for example >> sheet 46 on: >> >> http://cs.ulb.ac.be/cours/info364/relnormnotes.pdf
>
> Did anyone notice that this algorithm is actually not correct? Take the
> following example of a relation R(A,B,C,D,E) with the set of FDs:
>
> { AB->C, AB->D, BC->D }
You've lost E - was that a mistake in the FD's or in the example relation?
> It is clear that the relation ABCD is not in 3NF. Since the set of FDs
> it is already a minimal cover the resulting decomposition is:
>
> { ABCD, BCD }
>
> But that gives us our old relation back (plus a projection) so this is
> definitely not in 3NF.
>
> The strange thing is that this algorithm appears as such in the Elmasri
> and Navathe and also in Date (but not Ullman). Surely these two major
> textbooks would not get the most fundamental algorithm in normalization
> theory wrong? Or would they? Reminds me a little of the
> misrepresentation of 5NF in many textbooks.
>
> -- Jan Hidders
-- Jonathan Leffler #include <disclaimer.h> Email: jleffler_at_earthlink.net, jleffler_at_us.ibm.com Guardian of DBD::Informix v2003.04 -- http://dbi.perl.org/Received on Sat Apr 10 2004 - 06:31:06 CEST