Re: thinking about UPDATE

"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. Received on Fri Jul 23 2004 - 00:22:31 CEST