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Re: Entity vs. Table

From: Alan <alan_at_erols.com>
Date: Tue, 15 Jun 2004 09:23:35 -0400
Message-ID: <2j8bioFu3trsU1@uni-berlin.de>

"Alfredo Novoa" <alfredo_at_ncs.es> wrote in message news:e4330f45.0406150243.6ba141da_at_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.

I am talking about taking a completed relational model design and implementing it. You know, I've designed all the tables and now I'm typing SQL to build them in the RDBMS.

>
> > > >> 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.

No one said that the ERD models ALL business requirements. For the 50th time, it models relationships among the data. For example, it does not model what data type an attribute is, just as it does not model the calcualtions behind a derived attribute. No one said an ERD is the only technique used to capture 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.
>

This does not belong on an ERD.

> > Then you are perfect. Congratulations.
>
> No, but I don't see value in ERD.
>
>

> Regards
Received on Tue Jun 15 2004 - 08:23:35 CDT

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