Re: two nasty schemata, union types and surrogate keys
Date: 22 Sep 2009 21:27:14 GMT
>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
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.
-- ReinierReceived on Tue Sep 22 2009 - 16:27:14 CDT