Re: Entity vs. Table

From: Alan <>
Date: Mon, 14 Jun 2004 11:15:22 -0400
Message-ID: <>

"Alfredo Novoa" <> wrote in message
> On Mon, 14 Jun 2004 09:58:53 -0400, "Alan" <> 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.


> >, but that's no
> >reason not to use it
> >at all. Most situations can be represented. Take a look at
> > which I posted
> >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 - 17:15:22 CEST

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