Re: atomic

Quoth paul c:
> F {C}
> _ ___
>
> Joe {Jack, Jill}
> Bob {}
>
> This might mean that Bob is the father of the empty set of children. As
> far as the predicate is concerned, this might be seen as contradictory
> because if Bob has no children, he can't be a father. (By
> "contradictory", I don't mean a contradiction within the database,
> rather that the second tuple and the predicate contradict each other.)
>
> For that matter if F is not a primary key, I could say:
>
> F {C}
> _ ___
>
> Joe {Jack, Jill}
> Joe {}
>
> which seems to have two such contradictions if such a thing is possible.
>
> If RVA's aren't allowed (and neither are nulls), I think such
> contradictions aren't possible.

It is nothing to do with RVAs. You could with as much justification say that the relations ((Joe was born in 1970), (Bob was born in 2009)) or ((Joe was born in 1970), (Joe was born in 1975)) are contradictory.

> I'm wondering are there applications where RVA values that are "empty"
> make sense or are such values just a curious by-product of RVA's?

Consider the standard example of relvars and keys. Though of course, empty RVA values are in some sense a by-product of RVAs... :)

-- 
Jon