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Re: Pizza Example

From: Anthony W. Youngman <wol_at_thewolery.demon.co.uk>
Date: Wed, 7 Apr 2004 01:35:19 +0100
Message-ID: <+N$oICIHz0cAFw8o@thewolery.demon.co.uk>


In message <DIidncafbowjK-_dRVn-uA_at_comcast.com>, Laconic2 <laconic2_at_comcast.net> writes
>> More to the point, it's structured in a manner the database can
>> understand, which isn't the case if the information is scattered across
>> multiple tables :-)
>
>This gets to the heart of the matter. If I can expand on this, I have two
>questions:
>
>First, does the relational data model scatter information across multiple
>relations?

What do you mean? If you have a repeating attribute, the relational model demands that you spread data about a single object across multiple tables.
>
>Second, if so, does the database fail to understand what the relational
>model has done?

Let's assume we have several types of object. All have repeating attributes. And all are related, some by a many-2-many relation. Can a relational database group the tables according to the object they describe?
>
>And here's a third question: can a relational model be reconstructed from
>the database schema? If not, why not?
>

Why not? If you're talking about a Pick schema, then the only reason you can't get a relational model is if the Pick designer didn't do his job properly.

As a Pick database designer, I would have one FILE (our equivalent of "table") per real-world object type. The data in this file *is* *normalised*. It's just that it's NFNF (non first normal form).

So. Imagine you've defined a view, in your relational database, that joins all tables representing an object. You then "list" (sorry I don't know the relational term) one object in your view. In your two-dimensional view, imagine that all duplicated values just "don't exist". You now have the equivalent of a Pick RECORD (a bit like your row). We don't duplicate a simple attribute because it doesn't make sense to do so - why list it repeatedly when it only exists once per object?

Cheers,
Wol

-- 
Anthony W. Youngman - wol at thewolery dot demon dot co dot uk
HEX wondered how much he should tell the Wizards. He felt it would not be a
good idea to burden them with too much input. Hex always thought of his reports
as Lies-to-People.
The Science of Discworld : (c) Terry Pratchett 1999
Received on Tue Apr 06 2004 - 19:35:19 CDT

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