我要评论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 | +-------+------+
此题，其本质就是赋值行号(需要注意分数相同的情景).
在实践过程中，初版答案如下所示：
# write your mysql query statement below
select
a.score as score,
(select count(distinct b.score) from scores b where b.score >= a.score) as rank
from scores a order by a.score desc;
此处，使用select count来统计行号，注意使用distinct来区分相同分数.
但是，此解题方案的效率较差，sql运行肯定是越快越好.
因此，在sql中引入变量来赋值行号，以替代耗时的select count操作.
# write your mysql query statement below
select
score,
@row := @row + (@pre <> (@pre := score)) as rank
from scores, (select @row := 0, @pre := -1) t
order by score desc;
此处，查询是在scores与临时表之间进行cross join.
此外，使用临时变量(@row,@pre)记录行号.
tips:
- 通过
@pre与当前score的比较，确定是否+1,需要注意mysql中的true/false为0/1); - 在
sql中,set/update语句中,=表示赋值；在set/update/select中,:=表示赋值，因此使用:=.
通过以上改进，mysql的运行效率得到了较大的提高.
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