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Re: Counting propositions

From: Paul <paul_at_test.com>
Date: Wed, 16 Jun 2004 23:49:52 +0100
Message-ID: <O74Ac.15992$NK4.2654639@stones.force9.net>


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>

A relation corresponds to a set of propositions that all follow the same predicate "template". So your predicate might be:

p(x,y,z)

where x,y,z are "variables" that range over values in domain sets X,Y,Z say. But only some values of x,y,z will be true. There will be |X|.|Y|.|Z| possible ways of substituting values in the predicate (where |X| is the number of elements in the set X).

The closed world assumption says that everything not in the database is false. And the "SELECT COUNT(*)" query is saying how many values of (x,y,z) there are for which p(x,y,z) is true.

Paul. Received on Wed Jun 16 2004 - 17:49:52 CDT

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