Re: thinking about UPDATE

"Mikito Harakiri" <mikharakiri_at_iahu.com> wrote in message news:QiXLc.58$X73.394_at_news.oracle.com...
>
> "D Guntermann" <guntermann_at_hotmail.com> wrote in message
> news:I19vBu.41M_at_news.boeing.com...
> >
> > "Mikito Harakiri" <mikharakiri_at_iahu.com> wrote in message
> > news:S2ULc.50$X73.342_at_news.oracle.com...
> > > "Marshall Spight" <mspight_at_dnai.com> wrote in message
> > > news:QIQLc.139454$a24.125173_at_attbi_s03...
> > > > First, the case where the key is zero attributes. In this case,
> > > > there is no project operation that can reduce the number of
> > > > attributes in the key, so again nothing to do.
> > >
> > > A key with zero attributes means that the relation is allowed to have
> one
> > > tuple only, right?
> > >
> > >
> > I might be missing something here, but can one have a relation variable
> > extensionally defined with one or more attributes where the key is the
> empty
> > set?
>
> Without loss of generality we can limit our scope to FDs with RHS set
> containing one column only. Then,
>
> {} -> {A.X}
>
> means
>
> not exists a1,a2 ( a1.x != a2.x )
>
> Since this is true for any column X, then it simply says "no duplicate a1
> and a2 are allowed" -- a relation with one tuple, at most.

Perhaps more intuitive reasoning may be the observation that the key property is monotonic:
1. Key containing all the columns of the relation is always valid. 2. Key containing some of the columns of the relation might be valid or not. Unlike cases #1, and #3 this is actually the interesting (or nontrivial) one.
3. Key containing none of the columns of the relation is almost never valid. Received on Fri Jul 23 2004 - 00:41:42 CEST