Re: Entity vs. Table
Date: Mon, 14 Jun 2004 11:15:22 -0400
"Alfredo Novoa" <alfredo_at_ncs.es> wrote in message
> 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.
> >, 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
> >another thread. This may convince you otherwise.
> As I expected the vast majority of the business rules can not be
> 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.
Maybe _you_ have that problem. I don't.
> >Not if the model is built properly and follows the rules of translation
> >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.
Received on Mon Jun 14 2004 - 17:15:22 CEST