Re: Who invitented the division operator in relational algebra and why?

"Amund Trovåg" <amund_at_texassibir.com> wrote in message news:b7gotv$2d85$1_at_toralf.uib.no...
> Hi DB-theorists!
>
> The division operator in relational algebra is not part of the minimal set
> of operators (select, project, union, minus and cartesian product). Join was
> added to relational algebra because it was a practical operator was used a
> lot in practise. Although join can be performed with a combination of the
> minimal set, it was a simpler way of doing the common operation that joining
> relations is.
>
> Does anyone have references to the first article where the division operator
> was "made"?

According to Date, it was Codd's 1972 paper "Relational Completness of Data Base Sublanguages"

> The division operator can make universal queries, but can it do
> all queries? Is it possible to formulate the query: 'Find employees that
> work on all the projects of its own department' with the division operator?
>
> I will be very greatful if someone has any pointers to some or all of these
> questions, as I am doing a literature study prior to a thesis project, and I
> am having some trouble finding out the why's of the division operator.

I would at the very least see what Date has to say about the divide operator in his Intro to DB Systems.

Regards
Paul Vernon
Business Intelligence, IBM Global Services Received on Tue Apr 15 2003 - 14:23:21 CEST