Post the problem statement and current code I am using, and wondering if any smart ideas to improve query performance? Using MySQL. Thanks.
Write a SQL query to rank scores. If there is a tie between two scores, both should have the same ranking. Note that after a tie, the next ranking number should be the next consecutive integer value. In other words, there should be no "holes" between ranks.
+----+-------+
| Id | Score |
+----+-------+
| 1 | 3.50 |
| 2 | 3.65 |
| 3 | 4.00 |
| 4 | 3.85 |
| 5 | 4.00 |
| 6 | 3.65 |
+----+-------+
For example, given the above Scores table, your query should generate the following report (order by highest score):
+-------+------+
| Score | Rank |
+-------+------+
| 4.00 | 1 |
| 4.00 | 1 |
| 3.85 | 2 |
| 3.65 | 3 |
| 3.65 | 3 |
| 3.50 | 4 |
+-------+------+
SELECT
s.score, scores_and_ranks.rank
FROM
Scores s
JOIN
(
SELECT
score_primary.score, COUNT(DISTINCT score_higher.score) + 1 AS rank
FROM
Scores score_primary
LEFT JOIN Scores score_higher
ON score_higher.score > score_primary.score
GROUP BY score_primary.score
) scores_and_ranks
ON s.score = scores_and_ranks.score
ORDER BY rank ASC;
BTW, post issue from Gordon's code.
BTW, tried sgeddes's code, but met with new issues,
New issue from Gordon's code,
thanks in advance, Lin
User defined variables are probably faster than what you are doing. However, you need to be careful when using them. In particular, you cannot assign a variable in one expression and use it in another -- I mean, you can, but the expressions can be evaluated in any order so your code may not do what you intend.
So, you need to do all the work in a single expression:
select s.*,
(@rn := if(@s = score, @rn,
if(@s := score, @rn + 1, @rn + 1)
)
) as rank
from scores s cross join
(select @rn := 0, @s := 0) params
order by score desc;
One option is to use user-defined variables
:
select score,
@rnk:=if(@prevScore=score,@rnk,@rnk+1) rnk,
@prevScore:=score
from scores
join (select @rnk:=0, @prevScore:=0) t
order by score desc
来源:https://stackoverflow.com/questions/34169402/sql-query-performance-improvement-for-advice