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Re: Entity vs. Table

From: Alfredo Novoa <alfredo_at_ncs.es>
Date: Mon, 14 Jun 2004 14:42:10 GMT
Message-ID: <40cdb920.19014080@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.

>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 in
>another thread. This may convince you otherwise.

As I expected the vast majority of the business rules can not be represented.

For instance this very simple rule: the stock of an article is the initial stock plus the inputs minus the outputs.

>> 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.

And what is the value added by the ERD?

I can start directly whith a relational design without wasting time with a very limited sketch.

Regards
  Alfredo Received on Mon Jun 14 2004 - 09:42:10 CDT

Original text of this message

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