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Re: Top n queries and GROUP BY

From: Rich Cullingford <rculling(at)sysd(dot)com>
To: Rich Cullingford <rculling(at)sysd(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Top n queries and GROUP BY
Date: 2003-11-17 17:56:23
Message-ID: 3FB90BC7.5090602@sysd.com (view raw or flat)
Thread:
Lists: pgsql-performance
Rich Cullingford wrote:
> All,
> This is a straight SQL question, maybe not appropriate for a performance 
> list, but...
> 
> I have a simple stock holdings setup:
> 
> => select * from t1;
>  nam |    co     | num
> -----+-----------+------
>  joe | ibm       |  600
>  abe | ibm       | 1500
>  joe | cisco     | 1200
>  abe | cisco     |  800
>  joe | novell    |  500
>  joe | microsoft |  200
> 
> What I would like to see is a Top-n-holdings-by-name", e.g, for n=2:
> 
>  nam      |  co    | num
> ----------+--------+-----
>  joe      | cisco  |  1200
>  joe      | ibm    |   600
>  abe      | ibm    |  1500
>  abe      | cisco  |   800
> 
> I can get part of the way by using a LIMIT clause in a subquery, e.g,
> 
> => select 'abe', a.co, a.num from (select co, num from t1 where 
> nam='abe' order by num desc limit 2) as a;
>  ?column? |  co   | num
> ----------+-------+------
>  abe      | ibm   | 1500
>  abe      | cisco |  800
> 
> but I can't figure out a correlated subquery (or GROUP BY arrangement or 
> anything else) that will cycle through the names. I vaguely remember 
> that these kinds or queries are hard to do in standard SQL, but I was 
> hoping that PG, with its extensions...

I forgot about row subqueries; for n=3, for example:

=> SELECT * FROM t1
      WHERE (nam,co,num) IN
                (SELECT nam,co,num FROM t1 b
                 where b.nam=t1.nam
                 order by num desc limit 3)
      order by nam, num desc;

  nam |   co   | num
-----+--------+------
  abe | ibm    | 1500
  abe | cisco  |  800
  joe | cisco  | 1200
  joe | ibm    |  600
  joe | novell |  500
(5 rows)

Seems to work...
                        Thanks all, Rich Cullingford
                                    rculling(at)sysd(dot)com


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