Re: Oids

"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 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) = {{}}

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 ) Received on Thu Aug 18 2005 - 00:02:50 CDT