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# Re: What is Aggregation? Re: grouping in tuple relational calculus

From: David Cressey <david.cressey_at_earthlink.net>
Date: Fri, 18 Feb 2005 13:40:10 GMT

"Mikito Harakiri" <mikharakiri_at_iahu.com> wrote in message news:nCaRd.42\$B47.156_at_news.oracle.com...
> Citing David Cressey: "Can you trust relational language specification to
> the users?"

You may be able to trust it to the users of the language, at least in part.

This is slightly off topic, but here goes:

A long time ago, I learned that there were 4 possible features of a set:

identity, order, interval, and proportion.

Every set we work with in IT has identity, but there may be sets that do not: (the set of all electrons?)
Order has been discussed endlessly (so far) in this NG. I will state that sometimes the representation has order, but the set represented does not. Interval is basically whether subtraction makes sense or not: (25 degrees celsius minus 18 degrees celsius).
Notice that "average" for temperatures is meaningful, although the "sum" is not.
Proportion is basically whether division makes sense for the set. It turns out that, if division makes sense, then so does addition. (distance, money).

The above is very, very informal. It's just to introduce the idea. I'll leave it up to the more formal denizens of the NG to express it more formally. Received on Fri Feb 18 2005 - 07:40:10 CST

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