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@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 Fri Apr 09 2004 - 23:31:06 CDT