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From: "Mikito Harakiri" <mikharakiri_nospaum@yahoo.com>
Newsgroups: comp.databases.theory
Subject: Re: Does Codd's view of a relational database differ from that ofDate&Darwin?[M.Gittens]
Date: 7 Jul 2005 09:11:23 -0700
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Xref: dp-news.maxwell.syr.edu comp.databases.theory:32080

Jon Heggland wrote:
> Because it isn't. Temperature is not the union of double, double and
> double, or even of fahrenheit, celsius and kelvin. Point is not the
> union of cartesian points and polar points. It does not even make sense
> to speak of such a union, because the set of cartesian points and the
> set of polar points is the same set---the set of 2D points. (0,4) and
> (4,90=B0) is the same point, just like 14 and 0xE is the same int.

User defined types present rather archaic approach to units. Modern
approach is symbolic manipulation

http://tinyurl.com/68rbm

(This is well known technique, BTW that has been posted many times on
sci.math).