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Re: Pizza Example

From: Alfredo Novoa <alfredo_at_ncs.es>
Date: Sat, 24 Apr 2004 14:23:49 GMT
Message-ID: <408a762d.3297481@news.wanadoo.es>


On Sat, 24 Apr 2004 09:45:20 -0400, "Laconic2" <laconic2_at_comcast.net> wrote:

>> But if you have an ER model you don't have a database design.
>
>That's the whole point. The ER model is useful for what it doesn't say, as
>much as for what it does say.

IMO it is unuseful and harmful because it can not say many things.

> It does establish the connection between the
>data, expressed in values and domains, and the subject matter, expressed
>in entities, relationships, and attributes.

It forces us to distinguish between entities and relationships without any sound criteria.

>It requires a little work to turn this into either a relational model or a
>CODASYL model, but you haven't hidden the design decisions in the ER model,
>where they are hard to see.

But the result is poor in most cases.

If you start directly designing a relational database, it is a lot easier to get a good design.

>>You can not turn a relational database design in a ER model in almost all
>cases.
>
>I didn't get this point. What are you saying?

That you can not represent most busines rules with an ER model. If you try to turn a relational database design into an ER model you will lose a lot of information.

Regards
  Alfredo Received on Sat Apr 24 2004 - 09:23:49 CDT

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