Re: Counting propositions

From: Dawn M. Wolthuis <>
Date: Mon, 14 Jun 2004 14:31:19 -0500
Message-ID: <cakuee$ak4$>

"x" <> wrote in message news:40cdceca$
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> "Jonathan Leffler" <> wrote in message
> news:lNjzc.160$
> > x wrote:
> > > "Laconic2" <> wrote:
> > >>"x" <> wrote:
> > >>> If tuples in a relational relation(ship) (or relvar) are
> > >>> propositions, what is the meaning of:
> > >>>
> > >>>select count(distinct *) as nr
> > >>>from <relvar>
> > >>>
> > >>>and the like ?
> Sorry for the distraction.
> I intended to ask what is the meaning of 'nr'

This is similar (not the same) to a question I have been planning to ask, so I'll toss it into your question, hopefully not altering the discussion too much. When, if ever, should aggregate values be designed into base relations?

Date uses a parts example, with a quantity, as an example relation in his database textbook. By designing a quantity of a part into a relation, there is an acknowledgement that you are designing aggregate data, rather than having a separate tuple for each real world part.

nr, in your example, is derived aggregate data. It is derived from the propositions already instantiated. Derived aggregate data is always with respect to a particular point in time, so that information needs to be in the resulting proposition in which nr is used.

At <point in time> the count of distinct instances of <relvar> is <nr>

I might be completely missing what you are getting at, however, so perhaps you could provide more clues?
--dawn Received on Mon Jun 14 2004 - 21:31:19 CEST

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