Re: a union is always a join!

>> Yes, but what I meant to say is that in general, the tuples
>> don't really express facts regarding those domain values,
>> they just help express information about other domain values.
>
>Tuples do not express facts about domain values. They contain domain
>values.

Duh.

True, I didn't express myself clearly enough.

But I don't need your lecturing. My point is that in a relational database, the tuples of a relation often correspond not just to the propositions of an associated predicate, but to observations; explicitly asserted, rather than derived information. To propositional logic, it's all the same, of course.

-- 
Reinier