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Home -> Community -> Usenet -> comp.databases.theory -> Re: Counting propositions
"Paul" <paul_at_test.com> wrote in message
news:O74Ac.15992$NK4.2654639_at_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.
Best answer so far.
So we aren't counting the propositions, we are counting the (x,y,z) values.
Why is this useful ?
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