Re: thinking about UPDATE

"D Guntermann" <guntermann_at_hotmail.com> wrote in message news:I1A396.CB5_at_news.boeing.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...
> > > 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.

There was a typo there: "only duplicate a1 and a2 are allowed"

> Perfectly understood! Thanks Mikito. Now, a follow-up:
>
> From {} -> {A.X} above, we could get: {} -> {{},A.X}.
>
> What are the implications of this?
>
> Would your predicate logic still hold?

This is really what "Without loss of generality" disclaimer took care of. On FDs Right Hand Side you can have any number of columns, but all these cases are trivial generalizations of the "canonic" case when you have only one column. Indeed,

{X,Y} -> {U}

and

{X,Y} -> {V}

imply

{X,Y} -> {U,V}

therefore, sets on the right side don't really buy us something.

(Aside from the simple fact, that allowing sets in place of columns is formally illegal in classic dependency theory). Received on Fri Jul 23 2004 - 02:47:13 CEST