In article <3FB8F98D.1010707@sysd.com>,
Rich Cullingford <rculling@sysd.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
