Re: The Practical Benefits of the Relational Model

Union, Intersection, Difference, Product, Restrict, Project, Join, Divide operators are considered the first of a set of formal relational operators (Rename can be considered also) in that they are operators on relations.

Why do you say that the Intersection is not a relational operator? Are you stating this because the intersection operator is not primitive in nature?

Daniel Guntermann

"Mikito Harakiri" <mikharakiri_at_yahoo.com> wrote in message news:bdf69bdf.0210041131.797ce5a9_at_posting.google.com...
> hidders_at_hcoss.uia.ac.be (Jan.Hidders) wrote in message
news:<3d9d91e8$1_at_news.uia.ac.be>...
> > In article <bdf69bdf.0210031023.375229d8_at_posting.google.com>,
> > Mikito Harakiri <mikharakiri_at_yahoo.com> wrote:
> > >
> > >It is often emphasized how beatuful relational theory is, because it
> > >is based on the set theory. While there is undoubtedly some
> > >connections, but may I ask why set union is a basic relational
> > >operator, and intersection is not? (Intersection could be expressed
> > >via combination of join and projection).
> >
> > Why do you ask questions and then put the answer right behind it in
> > brackets? :-)
> >
> > -- Jan Hidders
>
> This was not an answer. Union and intersection are dual operations in
> the set theory. Suddenly, this symmetry is broken in the relational
> theory.
Received on Fri Oct 04 2002 - 17:03:48 CDT