Re: two nasty schemata, union types and surrogate keys

Brian wrote:

>You're wrong, of course, but don't take my word for it. According to
>Date in /An Introduction to Database Systems, Eighth Edition/, page
>161: 'the Closed World Assumption (also known as the Closed World
>Interpretation) says that if an otherwise valid tuple--that is, one
>that conforms to the relvar heading--does /not/ appear in the body of
>the relvar, then we can assume the corresponding proposition is
>false.

But what *is* that proposition? It might be

  FIRTNAME LASTNAME is an employee at CORPORATION

but it might just as well be

  at some time in the past, it has been asserted that   FIRTNAME LASTNAME is an employee at CORPORATION   

which is a closed world formulation of what is approximately the open world counterpart of the first.

So just stating the closed world assumption isn't enough - you also have to rule out predicates that involve statements about assertions.

-- 
Reinier