Re: two nasty schemata, union types and surrogate keys

Brian wrote:

>Under the open world interpretation, tuples that can be in a relation
>but aren't represent propositions that may or may not be true. In
>other words, it is unknown whether those propositions are true or
>false, but for the tuples that are in a relation, it is not unknown
>whether the propositions represented are true because they are in fact
>supposed to be true.

They are supposed to be true, not known to be true. A database relation cannot be guaranteed to express facts, It expresses statements of fact, given an interpretation as a predicate. Whether this interpretation follows CWA or not doesn't make a difference.

>It follows, therefore, since it is not unknown
>whether the propositions represented are true, that they are known to
>be true.

But this is no different from the tuples in a relation interpreted under the closed world assumption.

>> >In other words, under the
>> >closed world interpretation, what is represented is supposed to be
>> >true, but under the open world interpretation, what is represented is
>> >only what is known to be true.

I still don't understand what difference between 'supposed' and 'known' you have in mind here.

-- 
Reinier