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Re: Entity vs. Table

From: Alan <alan_at_erols.com>
Date: Mon, 14 Jun 2004 11:15:22 -0400
Message-ID: <2j5to6FtlfulU1@uni-berlin.de>

"Alfredo Novoa" <alfredo_at_ncs.es> wrote in message news:40cdb920.19014080_at_news.wanadoo.es...
> On Mon, 14 Jun 2004 09:58:53 -0400, "Alan" <alan_at_erols.com> wrote:
>
> >> > Yes and no. There should be no redundancy
> >>
> >> Why not?
> >>
> >> >, or it is not a propely
> >> > implemented 3NF relational database. Otherwise, yes.
> >>
> >> 3NF has nothing to do with the physical level.
> >
> >Again, I am talking about an implementation of a 3NF logical design in a
> >physical model.
>
> 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. You know that, now come on...

>
> >Yes, there are some situations that an ERD can't represent
>
> Almost all practical designs.

Nonsense.

>
> >, but that's no
> >reason not to use it
> >at all. Most situations can be represented. Take a look at
> >http://www.cis.drexel.edu/faculty/song/605/AppendixA.PDF which I posted
in
> >another thread. This may convince you otherwise.
>
> As I expected the vast majority of the business rules can not be
> represented.
>

"Vast majority" is an awful lot. And wrong. If it was true, the ERD would have been scrapped years ago.

> 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.

> >> It is very incomplete and often leads to bad designs.

Maybe _you_ have that problem. I don't.

> >
> >Not if the model is built properly and follows the rules of translation
to a
> >relational schema, which no one here seems to know about.
>
> And what is the value added by the ERD?

See next comment.

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

>
> Regards
> Alfredo
Received on Mon Jun 14 2004 - 10:15:22 CDT

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