Subject: Re: pg, mysql comparison with "group by" clause
From: Scott Marlowe <smarlowe@g2switchworks.com>
To: Greg Stark <gsstark@mit.edu>
Cc: Stephan Szabo <sszabo@megazone.bigpanda.com>,
	Rick Schumeyer <rschumeyer@ieee.org>, pgsql-sql@postgresql.org
In-Reply-To: <87br1u1jvq.fsf@stark.xeocode.com>
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	<20051011142658.C54076@megazone.bigpanda.com>
	<87br1u1jvq.fsf@stark.xeocode.com>
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Date: Wed, 12 Oct 2005 17:24:35 -0500

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. :)