Re: UNION ALL and SELECT DISTINCT

"dawn" <dawnwolthuis_at_gmail.com> wrote in message news:1144250458.682759.94210_at_e56g2000cwe.googlegroups.com...
> David Cressey wrote:
> > The nice folks who gave us the SQL language adopted two different
> > conventions regarding duplicates in result tables.
> >
> > In the SELECT operator, they chose to have SELECT mean the same as
SELECT
> > ALL. If you want SELECT DISTINCT, you have to say so.
> >
> > Notice that UNION is just the reverse. If you want UNION ALL, you have
to
> > say so. If you just say UNION, you get only distinct rows in the
result.
> >
> > I think it would have been nicer if they had adopted the same convention
in
> > both cases.
>
> I might not be thinking clearly, but I think the choices they made make
> some sense to me. I want the selection and projection to be separate.
> If I add one attribute to a view, for example, I would not want the
> number of rows to change just because now what used to be a duplicate
> row is unique. Or am I missing the point?

You misunderstood what I was asking for. I was not asking to eliminate the difference between SELECT ALL and SELECT DISTINCT. All I was asking for is that SELECT should default to SELECT DISTINCT. When making views you could still specify either one.

> SQL is a relatively dead language anyway

Ha!
> I would guess that most would agree that they would expect accurate and
> optimize on speed.

  You might be surprised. There are MANY times when I've suggested a SELECT DISTINCT to a neophyte, only to hear him wail ("but that TAKES TOO LONG".) They are missing the point.

If SELECT DISTINCT gives correct results, but SELECT ALL gives incoreect results, it's time to stop and think about the nature of the logical query. The correct way to optimize is to first come up with a class of queries that give the right answer, and then find one that runs fast.

There a large any numberof neophytes who try to do just the opposite: they try to discover all the queries that run fast. Then, among those, they try to pick one that gives the right results! This sounds absurd, but I speak from direct experience. Received on Wed Apr 05 2006 - 23:05:40 CEST