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Re: Entity vs. Table

From: Alfredo Novoa <alfredo_at_ncs.es>
Date: 15 Jun 2004 03:43:10 -0700
Message-ID: <e4330f45.0406150243.6ba141da@posting.google.com>


"Alan" <not.me_at_uhuh.rcn.com> wrote in message news:<b7qzc.27464$wi2.9450_at_nwrdny01.gnilink.net>...

> > If you mean that there should not be redundancy in the physical level
> > it is a striking statemen!
>
> Not what I mean at all. "Properly implemented" means tables that contain the
> proper attributes.

I don't follow you.

The Relational Model says nothing about the physical level.

> > >> 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.
> >
> > How?
> >
>
> Depends on the taxonomy you use. The one I am familiar with works like this:
>
> An entity is represented by a rectangle (of course).
> "Attached" to the rectangle are the attributes. These are encased in small
> ovals.
> Derived attributes (such as an aggregation) are encased in a double oval.

But where is the rule?

A double oval says that an attribute is derived but it says nothing about the derivation rules.

You are losing the business requirements.

> > Another example: No supplier with status less than 20 supplies any
> > part in a product with quality extra.
>
> I am not sure what you mean here (language problem).

You can not use parts from not very confidable suppliers in very high quality products.

> Then you are perfect. Congratulations.

No, but I don't see value in ERD.

Regards Received on Tue Jun 15 2004 - 05:43:10 CDT

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