Re: Counting propositions

From: x <>
Date: Tue, 15 Jun 2004 15:32:29 +0300
Message-ID: <40ceeb61$>

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"Dawn M. Wolthuis" <> wrote in message news:cakuee$ak4$

> When, if ever, should aggregate values be designed into base
> relations?

This is a tough one. :-)
Because being a base or a derived relation is accidental there is no rule. :-)
The only rule I can think of is minimizing consistency checks.

> 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,
> is an acknowledgement that you are designing aggregate data, rather than
> having a separate tuple for each real world part.

When there is no way to discriminate the "real world parts" by data in the database.
 Think about this. If you think you understood, think again.

> 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.

Both derived and base aggregate data depend on the point in time.

> 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?

Why someone would need to count "distinct instances of <relvar>" ?


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Received on Tue Jun 15 2004 - 14:32:29 CEST

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