Re: constraints in algebra instead of calculus

From: David Cressey <cressey73_at_verizon.net>
Date: Tue, 19 Jun 2007 06:26:24 GMT
Message-ID: <kGKdi.6495$Fw5.6481@trndny02>

"paul c" <toledobythesea_at_oohay.ac> wrote in message news:vNFdi.37701$NV3.32514_at_pd7urf2no...
> Bob Badour wrote:
> > David Cressey wrote:
> >
> >> "paul c" <toledobythesea_at_oohay.ac> wrote in message
> >> news:MtUci.31107$NV3.16822_at_pd7urf2no...
> >>
> >>> paul c wrote:
> >>> ...
> >>>
> >>>> ... In other words, when all
> >>>> attributes are grouped, the result has the same number of rows as the
> >>>> input.
> >>>> ...
> >>>
> >>>
> >>> If this is right, what I don't see is how one could GROUP a table with
> >>> more more than one row and get a result that had only one row unless
> >>> some projection to eliminate an attribute was made after grouping on
all
> >>> attributes except the one that is subsequently projected away. Not
sure
> >>> if that matters though.
> >>
> >>
> >>
> >> PMFJI. I'm not following much of this discussion, but I want to ask
the
> >> following.
> >>
> >> If you were to GROUP on no attributes at all, wouldn't you get a
result
> >> with only one row?
> >> Does "GROUP on no attributes at all" even make sense?
> >
> >
> > That would extend each tuple with a named attribute equal to DEE. So,
> > yes, the relation value attribute added to each row would have one row.
> > But the cardinality of the derived relation would equal the cardinality
> > of the original relation.

> What I wrote above was wrong-headed, I had the TTM GROUP def'n all
> balled-up. I guess any time some operation involves sets it makes
> "sense" to check what happens when all of the set or none of it is
> specified, whether the result is useful is another thing. Grouping on
> all attributes seems to have occasional use but at the moment I can't
> seem much use for grouping on none.

I can't see much use for grouping on all attributes. It seems to me that this has to be a null operation.
>
> p
Received on Tue Jun 19 2007 - 01:26:24 CDT