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Re: Which normal form is this violating?

From: Jan.Hidders <hidders_at_uia.ua.ac.be>
Date: Mon, 6 May 2002 19:08:21 +0200
Message-ID: <Pine.GSO.4.40.0205061824330.17626-100000@hcoss.uia.ac.be>


On Mon, 6 May 2002, Daniel Guntermann wrote:
> Jan Hidders wrote:
> >
> > It is indeed true that a schema should not describe two tables
> > that are logically the same, i.e., represent the same predicate,
> > because then one table would be redundant.
>
> This statement seems to imply any horizontal partitioning,
> snapshots, materialized views, or several other types of conventions
> used in databases are now invalidated.

If you partition a table horizontally you get tables that represent different predicates (if the original predicate was P(x) then the new predicates are "P(x) AND x.city = 'Paris'" and "P(x) AND x.city = 'New-York'" et cetera). If you take a snapshot you also get a predicate different from P(x) : "P(x) holds at moment t". So you see that it is not so obvious that this will give you (in a useful way) two tables that represent exactly the same predicate.

> Also, it seems that most textbooks I've come across, stress the fact
> that the relational model does a great deal to "reduce" redundancy
> (Maybe it says differently in Celko's book). They have never
> claimed to say that redundany is entirely eliminated, nor do they
> say that this is always desirable.

Indeed. But what would be the point of having two tables in your schema that are always exactly the same?

Received on Mon May 06 2002 - 12:08:21 CDT

Original text of this message

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