Re: two nasty schemata, union types and surrogate keys
Date: Sun, 1 Nov 2009 06:37:44 -0800 (PST)
On Oct 31, 8:36 am, r..._at_raampje.lan (Reinier Post) wrote:
> 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.
Under the closed world interpretation, there are no unknown truth values, but under the open world interpretation, only what has been explicitly asserted is known to be true, and it is known to be true even if the user that made the assertion is mistaken. One has to assume--especially if the identity of the user is not also being recorded--that what the users assert is true to the best of their knowledge. (If the identity of the user /is/ being recorded, then what is being recorded is not just known or suppposed to be true: it actually is true--even if the user is lying.) Just because something is known to be true doesn't mean that it actually is true. Even under the closed world interpretation, what is supposed to be true may not actually be true. Received on Sun Nov 01 2009 - 15:37:44 CET