Re: Why spurious tuples with fifth normal form?

From: Jan Hidders <>
Date: Wed, 15 Jun 2005 22:56:22 GMT
Message-ID: <qm2se.121797$> wrote:
> I have spent countless hours searching the net to find the answer, but
> am still confused. I have a db structure where I have decomposed a
> table into three smaller tables, but when I join them all back
> together, I get spurious tuples. Now I thought that the whole point of
> fifth normal form was to allow you to successfully decompose a table
> into smaller tables, then join them back together without spurious
> results. Do I have the wrong idea about this, or if not, what am I
> doing wrong?

You are only supposed to split if there is a join-dependency that does not follow from the key constraints. If this is not the case then you are already in 5NF. It sounds like you are decomposing your relation without ever checking if there was a join-dependency that required this. In fact, if there are spurious tuples then that actually proves (pretty much by definition) that your decomposition does not correspond to a join-dependency.

  • Jan Hidders
Received on Thu Jun 16 2005 - 00:56:22 CEST

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