Re: 4 the FAQ: Are Commercial DBMS Truly Relational?
Date: Sun, 10 Oct 2004 15:23:28 GMT
Message-ID: <Qtcad.366672$Fg5.150351_at_attbi_s53>
"Kenneth Downs" <firstinit.lastname_at_lastnameplusfam.net> wrote in message news:pbhbkc.5d7.ln_at_mercury.downsfam.net...
> Marshall Spight wrote:
>
> > "Christopher Browne" <cbbrowne_at_acm.org> wrote in message
> > news:2srmk1F1njk29U1_at_uni-berlin.de...
> >>
> >> If you cut columns off of the result set, it is possible for the
> >> result set to, in fact, not be a "set", but rather a non-unique "bag"
> >> of tuples.
> >> [...]
> >> That is NOT going to be a "set" or a "relation" if some customer made
> >> multiple purchases between those dates.
> >>
> >> The problem that this expresses is that the relational algebra does
> >> not satisfy the property of closure.
> >
> > That's an odd viewpoint.
> >
> > The math books I've read have made the point that when you
> > are talking about sets, then {2, 2} is the same set as {2}.
> > So if you have duplicates after a project, you just throw
> > them away; they "don't count" so to speak.
> >
>
> Therefore, wouldn't the implementation of SQL have some sort of implied
> DISTINCT on every SELECT statement?