Re: Who invitented the division operator in relational algebra and why?
Date: Tue, 15 Apr 2003 15:39:53 +0200
Message-ID: <3e9c0bc6$1_at_news.broadpark.no>
"Paul Vernon" <paul.vernon_at_ukk.ibmm.comm> wrote in message
news:b7gtnc$1tvm$1_at_gazette.almaden.ibm.com...
> "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"
Excellent, thank you.
>
> > 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.
I will look into that one, again thanks very much :)
> Regards
> Paul Vernon
> Business Intelligence, IBM Global Services
>
>
Received on Tue Apr 15 2003 - 15:39:53 CEST