Re: two nasty schemata, union types and surrogate keys
Date: Fri, 25 Sep 2009 23:45:51 GMT
> ... 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?
Also, do views/derived relvars, eg., joins and unions, have complements that could theoretically be recorded? Received on Fri Sep 25 2009 - 18:45:51 CDT