Re: Oids

VC wrote:
> "Mikito Harakiri" <mikharakiri_nospaum_at_yahoo.com> wrote in message
> news:1124335766.107730.187550_at_z14g2000cwz.googlegroups.com...
> > VC wrote:
> >> project({}, A) = 0x, where x is either 0 or 1 depending on whether A is
> >> empty or not (by the projection definition)
> >
> > Could you please supply a reference to projection definition, which is
> > general enough to cover the empty set of attributes case?
> >
>
> 1. I do not have D&D, but the definition goes like this (the relation
> body):
>
> Project(A, R) = { {(a, v) }| (a, v) in t, a in A}| t in R} where R is a

I'm sorry, but the curly brackets aren't balanced.

> relation, A is a set of attributes to project over, a,v,t are attribute,
> value, t variables respectively. Clearly, if A ={}, then Project({}, R) =
> {{}}

Therefore, Project({}, R) = {{}} for any R ?

> 2. Ipse dixit:
>
> "The corresponding relation would be given by SP{ } (the projection of SP
> over no attributes). SP{ } evaluates to either TABLE_DEE or TABLE_DUM,
> according to the existence or not of at least one tuple in SP.
> "
> ( http://www.dbdebunk.com/page/page/1043196.htm )

Well, I see the proposition

Project({}, R) = DEE or DUM

there, but fail to see the formal argument for it. Received on Thu Aug 18 2005 - 18:45:34 CEST