Re: Aggregation is Inverse of Join

Marshall wrote:
> On Jan 15, 8:41 am, "Walt" <wami..._at_verizon.net> wrote:
> > "Marshall" <marshall.spi..._at_gmail.com> wrote in
> >
> > > Well, I've run out of steam. More later.
> >
> > > Marshall
> >
> > > PS. I told you I wasn't organized.
>
> > Interesting. It's going to take me weeks to get it.
> >
> > In the meantime, what is the inverse of Project?
>
> I think the usual term is "extend."
>
> In the relational lattice, project is just a specialization of the
> inner union, with one of the operands having cardinality zero.

Perhaps, "extend" is dual of "project", not inverse? Indeed the project is a union, and the extend is a join (of a relation with a function). Join and union are dual, not inverse of each others. It is the antijoin which is inverse of union... Received on Mon Jan 15 2007 - 21:25:28 CET