Re: Entity vs. Table

From: Alfredo Novoa <>
Date: Mon, 14 Jun 2004 17:19:31 GMT
Message-ID: <>

On Mon, 14 Jun 2004 11:15:22 -0400, "Alan" <> wrote:

>> So, an implementation of a 3NF logical design migth have redundancy.
>Those are your words, not mine. There should be no redundancy in a properly
>implemeted 3NF design

If you mean that there should not be redundancy in the physical level it is a striking statemen!

Why not?

All the books I readed say the contrary.

> And wrong. If it was true, the ERD would
>have been scrapped years ago.

ERD was scrapped decades ago by many people, but many poor practices survive for a very long time.

>> For instance this very simple rule: the stock of an article is the
>> initial stock plus the inputs minus the outputs.
>That is an aggregation and fits on an ERD with no problem.


> The underlying
>calculation does not belong on an ERD- it is an implementation detail.

My example is a logical integrity rule. The implementation is irrelevant in this context. We are talking about the logical model.

That rule is very easy to declare with a relational language.

Another example: No supplier with status less than 20 supplies any part in a product with quality extra.

>> I can start directly whith a relational design without wasting time
>> with a very limited sketch.
>So can I, and I often do, but I've also been burnt by not creating an ERD.

I never had problems.

>Relationships among data that you may not otherwise anticipate often reveal
>themselves in an ERD, especially with specialization/generalization.

Relationships among data are database relations also known as tables.

The relational model is the best way to represent relationships among data, and it does not have the problems of ERD, like the arbitrary and artificial distinction among entities and relationships (there are only relationships), and it has a lot greater expresive power.

  Alfredo Received on Mon Jun 14 2004 - 19:19:31 CEST

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