SQL query performance improvement for advice

问题

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

回答1:

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;

回答2:

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

SQL Fiddle Demo

来源：https://stackoverflow.com/questions/34169402/sql-query-performance-improvement-for-advice

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performance

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