Date: Thu, 5 Aug 2004 22:22:33 -0700 (PDT)
From: Stephan Szabo <sszabo@megazone.bigpanda.com>
To: Ram Nathaniel <ram_nathaniel@hotmail.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: SQL syntax extentions - to put postgres ahead in the race
In-Reply-To: <BAY7-DAV12V0KUvkRkZ000f7e18@hotmail.com>
Message-ID: <20040805220147.O92438@megazone.bigpanda.com>
References: <BAY7-DAV12V0KUvkRkZ000f7e18@hotmail.com>
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On Thu, 5 Aug 2004, Ram Nathaniel wrote:

>
> 1) The operator  "of max":
> suppose I have a table "grades" of 3 fields: class/student/grade where I
> store many grades of many students of many classes. I want to get the
> name of the highest scoring student in each class. Note that there may
> be many students with the same grade, but for starters let's say there
> is a primary key of class+grade.
>
> My query would now be:
> select student from grades where class+'#'+grade in
> (
>    select class+'#'+max(grade) from grades group by class
> ) a

As a side note, I'd think that something like:
 select student from grades where (class,grade) in
  (select class, max(grade) from grades group by class);
should avoid textual operations.  I'm assuming the + above are meant to be
concatenation (||).

> The optimal would be to introduce a new operator "of max" that would be used as follows:
>
> select student of max(grade) from grades group by class

PostgreSQL provides an extension called DISTINCT ON.

Something like
 select distinct on (class) student from grades order by class, grade
 desc;
should get you one arbitrary student with the highest grade in his or her
class.

If you want to order by the grades, I think you need a layer around it.
If you don't care about the class order, you might consider making the
class ordering desc as well to make it easier to use a multi-column index
on (class,grade).

> 2) aggregated concatenation:

Theoretically, you should be able to do this right now in PostgreSQL with
user defined aggregates (although you can't pass a second argument
currently for the separator).  I believe that an ordered subquery in FROM
will currently allow you to get an ordered aggregate, or perhaps you'd
have to turn off hash aggregation, but I think you should be able to get
it to keep the ordering.