Re: thinking about UPDATE
Date: Fri, 23 Jul 2004 00:21:45 GMT
Message-ID: <I1A3o9.CHr_at_news.boeing.com>
"Mikito Harakiri" <mikharakiri_at_iahu.com> wrote in message
news:QAXLc.59$X73.404_at_news.oracle.com...
> "Mikito Harakiri" <mikharakiri_at_iahu.com> wrote in message
> news:QiXLc.58$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.
Hi Mikito,
Would you mind clarifying what you mean be "almost never valid" in rule 3, above? Is there a way or criterion to discern when it is and when it is not valid?
- Dan > >