Re: The Practical Benefits of the Relational Model

By "set union" I can only assume you mean relational union because the relational operators, operate on realtions. I don't have a copy of RM/V2 in front of me so I can't guarantee I'm not delusional but I remember relational intersection as being in there.

Tod

"Mikito Harakiri" <mikharakiri_at_yahoo.com> wrote in message news:bdf69bdf.0210031023.375229d8_at_posting.google.com...
> Lauri Pietarinen <lauri.pietarinen_at_atbusiness.com> wrote in message
news:<3D9BE382.A470F58C_at_atbusiness.com>...
> > David Cressey wrote:
> >
> > > Don't expect people to stop saying that "A real relational system has
never
> > > been implemented". You will just be disappointed.
> >
> > Strangely enough, a "real" relational system _has_ been implemented -
and that
> > was a long time ago. See
> >
> > http://www.mcjones.org/System_R/bs12.html
> >
> > Obviously SQL does not even _try_ to be relational, and never has
> > tried to. I am referring here to the fact that result tables can have
> > duplicate rows.
> > By the way, the SQL1999-standard does not even mention the word
> > "relational".
>
> What relational purity really buys us? Unability to express
> aggregation? Ignoring nulls? Not admiting nested subqueries? Total
> ignorance about domain operators? God forbid nesting/unnesting!?
> Recursion impotence?
>
> 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).
Received on Fri Oct 04 2002 - 00:18:17 CEST