Re: two nasty schemata, union types and surrogate keys
Date: Sat, 26 Sep 2009 17:33:38 -0700 (PDT)
On Sep 25, 7:45 pm, paul c <toledobythe..._at_oohay.ac> wrote:
> Brian wrote:
> > ... 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. In other words, the body of the relvar at any given time
> > contains /all/ and /only/ the tuples that correspond to true
> > propositions at that time.' So the closed world assumption tells us
> > that what is actually in the database is supposed to be true, while
> > what is not is supposed to be false.
> > ...
> Regarding the actual quote, I've long taken it to mean also that if a
> relvar's complement were recorded and an otherwise valid tuple did not
> appear in the complement, then it must appear in the body of the relvar.
> Am I right?
That's the basic idea.
> Also, do views/derived relvars, eg., joins and unions, have complements
> that could theoretically be recorded?
The complement of a union is exactly those tuples that can theoretically appear in either operand but don't; the complement of a join is exactly those tuples that can theoretically appear in the result but don't. Received on Sat Sep 26 2009 - 19:33:38 CDT