Re: two nasty schemata, union types and surrogate keys

>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

Yes ...

>even if the user that made the assertion is mistaken.

Huh?! So if I have a database relation 'X works at Y', with the open world assumption, if someone updates the relation to say that Jane Doe works at Acme Corp., then I must assume this to be true even if that person is mistaken? How does that make any sense?

>[...] 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.

Exactly. There is no difference between OWA and CWA in this respect, regarding the assertions that correspond to tuples in the relation. The difference is in what they imply for assertions that correspond to tuples not in the relatoion.

-- 
Reinier