Re: Entity vs. Table

• Original Message ----- From: "Alfredo Novoa" <alfredo_at_ncs.es> Newsgroups: comp.databases.theory Sent: Monday, June 14, 2004 9:27 AM Subject: Re: Entity vs. Table

> "Alan" <alan_at_erols.com> wrote in message
news:<2ittqgFqitf3U1_at_uni-berlin.de>...
>
> > It's not a requirement, but it is a starting point. If you start with a
3NF
> > physical model, you can denormalize for the sake of performance.
>
> Physical designs can't be 3NF. Normalization only applies to the
> logical level.
>

I am talking about an implementation of a 3NF logical design in a physical model. I didn't state it clearly.

> > > A TRY :
> > > Physical datamodel is a model of the
> > > database implementation ? (The databasemodel).
> > > (Can contain redundancy does not have to be like the
> > > logical datamodel, but MUST implement the logical model).
> >
> > 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.

>
> > A logical data model follows after the ERD, so I would say no to this
part.
>
> You can not represent many business rules with an ERD. IMO the ERD is
> something we should avoid.

Yes, there are some situations that an ERD can't represent, 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.

>
> > Yes, with one small change. An ERD is a conceptual business model of the
> > data and the realtionships(associations) among the data.
>
> It is very incomplete and often leads to bad designs.

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.

>
>
> Regards
> Alfredo
Received on Mon Jun 14 2004 - 15:58:53 CEST