Re: Multi-Valued Dependencies
Date: 2000/03/22
Message-ID: <828C4.1875$U95.25083_at_news.pacbell.net>#1/1
Randy Yates wrote in message <38D8B9BC.A4588176_at_207.87.184.178>...
>OK, now here is the issue I have: At this point, Date asserts that
>the course/teachers and course/texts relvars have no MVDs. I disagree.
>Consider the course/teachers relvar, and let A=COURSE, B=TEACHER,
>and C=nullset. Then A ->-> B.
Perhaps Date should have said that the relvars have no *non-trivial* MVDs, or amended the definition of MVD to require the attribute sets B and C to be non-empty.
You are technically correct that COURSE ->-> TEACHER in the given relation. In fact, in *any* relation with two or more attributes, you can partition the attributes into subsets X and Y and verify that X ->-> Y. This fact is not very interesting though. When designers talk about MVDs, they generally mean non-trivial ones. Received on Wed Mar 22 2000 - 00:00:00 CET