Re: pg, mysql comparison with "group by" clause - Mailing list pgsql-sql

> Not always, but I'd rather get the right answer with difficulty than
the
wrong one with ease. :)

agreed.

I made it a point to mention this so called "feature" in my book.

This is a bug they never fixed and they decided to call it a feature.

It is, imo, *ridiculous*.

Regards, Anthony

-----Original Message-----
From: pgsql-sql-owner@postgresql.org
[mailto:pgsql-sql-owner@postgresql.org] On Behalf Of Scott Marlowe
Sent: Wednesday, October 12, 2005 6:25 PM
To: Greg Stark
Cc: Stephan Szabo; Rick Schumeyer; pgsql-sql@postgresql.org
Subject: Re: [SQL] pg, mysql comparison with "group by" clause

On Wed, 2005-10-12 at 16:54, Greg Stark wrote:
> Stephan Szabo <sszabo@megazone.bigpanda.com> writes:
>
> > On Tue, 11 Oct 2005, Rick Schumeyer wrote:
> >
> > > I'm not sure what I was thinking, but I tried the following query
in pg:
> > >
> > > SELECT * FROM t GROUP BY state;
> > >
> > > pg returns an error.
> > >
> > > Mysql, OTOH, returns the first row for each state.  (The first row
with
> > > "AK", the first row with "PA", etc.)
> > >
> > > I'm no SQL expert, but it seems to me that the pg behavior is
correct, and
> > > the mysql result is just weird.  Am I correct?
> >
> > In your case, it sounds like the mysql result is wrong. I believe
SQL99
> > would allow it if the other columns were functionally dependant upon
state
> > (as there'd by definition only be one value for the other columns
per
> > group).
>
> I believe this is a documented feature.

Hehe.  When I turn on my windshield wipers and my airbag deploys, is it
a documented "feature" if the dealership told me about this behaviour
ahead of time?  In much the same way, while this behaviour may be
documented by MySQL, I can't imagine it really being called a feature.
But at least this misbehaviour is documented.  However, I think most
people in the MySQL universe just stumble onto it by accident when they
try it and it works.  I'd at least prefer it to throw a warning or
notice or something.

> MySQL treats "select a,b from t group by a" equivalently to Postgres's

> "select distinct on (a) a,b from t"
>
> I suppose "equivalent" isn't quite true. It's more general since it
allows
> aggregate functions as well. The equivalently general Postgres syntax
is to
> have a first() aggregate function and do "select a,first(b) from t
group by a".

A Subselect would let you do such a thing as well, and while it's more
complicated to write, it is likely to be easier to tell just what it's
doing.

> I'm sure it's very convenient.

Not always, but I'd rather get the right answer with difficulty than the
wrong one with ease. :)

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