Re: SQL (was: Why using "Group By")
From: Tibor Karaszi <tibor_not_pressed_ham_.karaszi_at_cornerstone.se>
Date: Fri, 14 Mar 2003 17:53:00 GMT
Message-ID: <0Goca.393$Du.2297_at_newsc.telia.net>
Date: Fri, 14 Mar 2003 17:53:00 GMT
Message-ID: <0Goca.393$Du.2297_at_newsc.telia.net>
-- Tibor Karaszi "Bob Badour" <bbadour_at_golden.net> wrote in message news:7occa.17$zx3.2161727_at_mantis.golden.net...Received on Fri Mar 14 2003 - 18:53:00 CET
> "Mikito Harakiri" <mikharakiri_at_ywho.com> wrote in message
> news:Y58ca.24$wV5.164_at_news.oracle.com...
> > "Bob Badour" <bbadour_at_golden.net> wrote in message
> > news:NY7ca.4$GY2.112751_at_mantis.golden.net...
> > > > In SQL there is a "select" clause, it corresponds to projection
> > operation
> > >
> > > Actually, it is a combination of extend, project and summarize.
> >
> > Trivially so for "extend" being inverse to "project". Not sure about
> > summarize. Summarize result extends the input relation with additional
> > "aggregate" column, and projecting the input relation to the "group by"
> > column at the same time. In the set symantics you have to say no more,
but
> > in the bug semantics I have to apply restriction to make rows
"distinct". >
> Were the latter semantics a freudian slip? Technically SQL's select list
> isn't even project unless it includes distinct or group by, but I didn't
> want to seem too contrary.
> >
> > > Table closure actually. Have you ever considered that SQL's cartesian
> > > product followed by restriction, if handled correctly, is a poor
cousin
> of
> > a
> > > relational join? Have you ever noted that it forces the effort to
> identify
> > > the common columns onto the user?
> >
> > Why is it such a big deal?
>
> Forcing users to perform easily automated tasks? Do you really have to
ask? ANSI SQL has a join type (NATURAL) where the engine automatically joins over common column names. I don't think that it is widely implemented, though. I'd prefer if we had such an option for FOREIGN KEYs.
> > > > Subquery
> > > > into the "select" clause, as we saw, is a nice way to express
> > aggregation.
> > > > Chris Date considers scalar subqueries disgusting, but are they
> really?
> > >
> > > Implicit conversion from a table to a scalar? Yeah, I agree they are
> > > disgusting.
> >
> > You wouldn't agree that any aggregate operation is a conversion of a
> > table/collection to a single value?
>
> You wouldn't agree that any aggregate operation is explicit?
> >
> > > > Finally, a
> > > > subquery in the "where" clause allows us to express logical
> quantifiers.
> > > > This is as much SQL as I'm able to digest so far, but I wasn't
> > > disappointed.
> > >
> > > Without logical identity, the first question that comes to mind is:
> > Logical
> > > quantifiers of what exactly?
> >
> > This idea needs more maturity, granted.
>
> Unfortunately, duplicates are in SQL to stay.
> >