Select top N rows out of M groups - mysql

Question

I have this table:
CREATE TABLE IF NOT EXISTS `catalog_sites` (
`id` int(10) unsigned NOT NULL auto_increment,
`cat_id` int(10) unsigned NOT NULL,
`date` datetime NOT NULL,
`url` varchar(255) NOT NULL,
`title` varchar(255) NOT NULL,
`description` varchar(255) NOT NULL,
`keywords` varchar(255) NOT NULL,
`visited` int(10) unsigned NOT NULL,
`shown` int(10) unsigned NOT NULL,
`meta_try` int(1) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `url` (`url`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;
I think my problem is simple, but cant seem to find an appropriate solution..
So, this is a table with web-sites, I would like to get 6 sites in 6 different categories (cat_id, total: 36 rows) with the highest rating for each category. The rating is calculated as visited / shown.
I should get 36 rows containing 6 top categories (we can find them by sorting with AVG(visited / shown) ), and 6 top sites in each of these 6 categories.
If you have any ideas how this might happen differently, please tell me.

Answer 1

This should get you what you want using MySQL Variables, the inner query will pre-calculate the rank of visited / shown, and using an order by by the condition you want... Per Category, the highest ranks... and then using #vars will keep the #RankSeq sequentially 1-? per category. From that Prequery (aliased PQ), the OUTER query just queries the PreQuery where the URL's Rank Sequence is <= 6
To further ensure you are only getting the top 6 categories, the inner PreQuery also has a pre-query / limit for the "TopCategories" (alias)
select
PQ.URL,
PQ.Cat_ID,
PQ.Rank,
PQ.URLRankSeq
from
( select
CS.cat_id,
(CS.visited / CS.shown ) as Rank,
CS.url,
#RankSeq := if( #LastCat = CS.Cat_ID, #RankSeq +1, 1 ) URLRankSeq,
#LastCat := CS.Cat_ID as ignoreIt
from
( select cat_id,
avg( visited / shown )
from catalog_sites
group by 1
order by 2 desc
limit 6 ) TopCategories
JOIN catalog_sites CS
on TopCategories.Cat_ID = CS.Cat_ID,
(select #RankSeq := 0, #LastCat = 0 ) SQLVars
order by
CS.cat_id,
Rank ) PQ
where
PQ.URLRankSeq <= 6

Answer 2

I've tried your example, but it doesn't really work for me, or I just don't know how to adapt it to my case. Anyway, I'm still a noob as far as SQL goes, so I couldn't understand your query.
I have managed to solve my problem however. It's complicated and probably the worst possible approach. It is slow too, but I'll cache the results, so that shouldn't be a problem.
Here is my solution:
SET #site_limit = 2;
SET #cat_limit = 6;
SET #row = 0;
SET #limiter = 0;
SET #last_cat = 0;
SELECT `cat_id`, `url`, `visited` / `shown` AS `rating`, #limiter := IF(#last_cat = `cat_id`, IF(#limiter >= #site_limit - 1, #limiter, #limiter + 1), 0) AS `limiter`, #last_cat := `cat_id` AS `last_cat`
FROM `catalog_sites`
WHERE `cat_id`
IN (
SELECT `cat_id`
FROM (
SELECT `cat_id`, #row := #row + 1 AS `row`
FROM (
SELECT `cat_id`
FROM `catalog_sites`
GROUP BY `cat_id`
ORDER BY AVG(`visited` / `shown`) DESC
) AS derived1
) AS derived2
WHERE `row` <= #cat_limit
)
GROUP BY `cat_id`, `limiter`
ORDER BY `cat_id`, `rating` DESC