Re: Entity vs. Table
Date: 15 Jun 2004 03:43:10 -0700
Message-ID: <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.
> > >> 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 - 12:43:10 CEST