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From: Keith H Duggar <duggar@alum.mit.edu>
Newsgroups: comp.databases.theory
Subject: Re: Declaring super types
Date: Sun, 25 Apr 2010 17:57:27 -0700 (PDT)
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On Apr 25, 3:02 pm, r...@raampje.lan (Reinier Post) wrote:
> Tegiri Nenashi wrote:
> >Second, why would I add redundant attributes to a Circle? If the idea
> >is to make both relations to have the same set of attributes, then we
> >go back to the previous paragraph: I'm interested to see a convincing
> >example of two relations with different sets of attributes that fits
> >your definition.
>
>   Person: first name, last name, date of birth
>   Citizen: first name, last name, date of birth, country of citizenship
>
> I've done some student instructions with that textbook and I still
> use the same ER modelling technique for myself; I've noticed that
> this is-a comes up pretty often, and that it is helpful, i.e. many

Does "is-a" come up because it follows naturally from the design
process? Or does it come up the same way that Object Oriented comes
up these days in programming discussions ie being shoe-horned into
the conversation whether needed or not?

How is this is-a concept "helpful" as you claim? For example, I
can't imagine myself every creating a database with the separate
Person and Citizen tables above.

> modelling errors I see can be explained in terms of "is-a being overlooked"
> or "is-a being modeled incorrectly".  It is also fairly common in tools.

And what happens if we simply banish "is-a" from our thinking and
vocabulary entirely? Are those modelling errors eliminated? What
do we lose by sacrificing this hierarchical notion?

KHD