mySQL Ranking (and draws) - mysql

Question

Next weekend we're having a competition with 3 qualifications a semifinal and a final. Only the best 15 participants could compete in the semifinal. Only the best 6 compete in the Finals.
in the qualifications you get a score from 0 to 100 for each qualification
I'm looking to find a way to select the contesters for the semi-final. This should be based on (rank of qualification1) * (rank of qualification2) * (rank of qualification3)
so i need something like:
select id, name, ((.... as RANK_OF_SCORE_1) * (.. as RANK_OF_SCORE_2) * (... as RANK_OF_SCORE_3)) as qualification_score from participants order by qualification_score desc limit 15
but of course this is not valid mySQL.
Besides this problem if tho contesters have the same score, they should be both included in the semi-finals even if this exceeds the maximum of 15.
For the finals, we would like to select the best 6 of the semi-final scores. If 2 scores are the same we would like to select on the qualifications..

Answer 1

option 1 : use postgres, which support windowing functions (namely RANK() and DENSE_RANK())
SELECT user_id, score, rank() over (order by score desc) from scores;
Time : 0.0014 s
option 2 : use a self- join : the rank of a user with score X is (1 +the count(*) of users with score less than X) ; this is likely to be pretty slow
CREATE TABLE scores( user_id INT PRIMARY KEY, score INT, KEY(score) );
INSERT INTO scores SELECT id, rand()*100 FROM serie LIMIT 1000;
SELECT a.user_id, a.score, 1+count(b.user_id) AS rank
FROM scores a
LEFT JOIN scores b ON (b.score>a.score)
GROUP BY user_id ORDER BY rank;
+---------+-------+------+
| user_id | score | rank |
+---------+-------+------+
| 381 | 100 | 1 |
| 777 | 100 | 1 |
| 586 | 100 | 1 |
| 907 | 100 | 1 |
| 790 | 100 | 1 |
| 253 | 99 | 6 |
| 393 | 99 | 6 |
| 429 | 99 | 6 |
| 376 | 99 | 6 |
| 857 | 99 | 6 |
| 293 | 99 | 6 |
| 156 | 99 | 6 |
| 167 | 98 | 13 |
| 594 | 98 | 13 |
| 690 | 98 | 13 |
| 510 | 98 | 13 |
| 436 | 98 | 13 |
| 671 | 98 | 13 |
time 0.7s
option 3 :
SET #rownum = 0;
SELECT a.user_id, a.score, b.r FROM
scores a
JOIN (
SELECT score, min(r) AS r FROM (
SELECT user_id, score, #rownum:=#rownum+1 AS r
FROM scores ORDER BY score DESC
) foo GROUP BY score
) b USING (score)
ORDER BY r;
time : 0.0014 s
EDIT
SET #rownum1 = 0;
SET #rownum2 = 0;
SET #rownum3 = 0;
SELECT s.*, s1.r, s2.r, s3.r FROM
scores s
JOIN
(
SELECT score_1, min(r) AS r FROM (
SELECT score_1, #rownum1:=#rownum1+1 AS r
FROM scores ORDER BY score_1 DESC
) foo GROUP BY score_1
) s1 USING (score_1) JOIN (
SELECT score_2, min(r) AS r FROM (
SELECT score_2, #rownum2:=#rownum2+1 AS r
FROM scores ORDER BY score_2 DESC
) foo GROUP BY score_2
) s2 USING (score_2) JOIN (
SELECT score_3, min(r) AS r FROM (
SELECT score_3, #rownum3:=#rownum3+1 AS r
FROM scores ORDER BY score_3 DESC
) foo GROUP BY score_3
) s3 USING (score_3)
ORDER BY s1.r * s2.r * s3.r;