Re: Top n queries and GROUP BY

In article <3FB8F98D(dot)1010707(at)sysd(dot)com>,
Rich Cullingford <rculling(at)sysd(dot)com> writes:

> 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...

How about an outer join?

SELECT x1.nam, x1.co, x1.num
FROM t1 x1
LEFT JOIN t1 x2 ON x2.nam = x1.nam AND x2.num > x1.num
GROUP BY x1.nam, x1.co, x1.num
HAVING count(*) < 2
ORDER BY x1.nam, x1.num DESC