mysql - 找出每组的最高 n 值

Question

我遇到了与此相同的问题Finding the highest n values of each group in MySQL

我有一些这样的数据:

+ --------- + ----------- +
| lane      | series      |
+ --------- + ----------- +
| 1         | 680         |
| 1         | 685         |
| 1         | 688         |
| 2         | 666         |
| 2         | 425         |
| 2         | 775         |
+ --------- + ----------- +

And I'd like to grab the highest n series per lane (let's say 2 for the sake of this example, but it could be many more than that) So the output should be:

+ --------- + ----------- +
| lane      | series      |
+ --------- + ----------- +
| 1         | 688         |
| 1         | 685         |
| 2         | 775         |
| 2         | 666         |
+ --------- + ----------- +

I think the SQL with "funky" MySQL features are quite good.

set @count:=-1, @lane:=0; 
select lane, series
from (select lane, series from lane_series order by lane, series desc) x
where if(lane != @lane, @count:=-1, 0) is not null
and if(lane != @lane, @lane:=lane, lane) is not null
and (@count:=@count+1) < 2; -- Specify the number of row at top of each group here

令我惊讶的是，它在 MySQL 5.0 和 MySQL 5.1 上运行得很好，但在我的生产 MySQL 版本 MySQL 5.5 上却不起作用。

MySQL 5.5上的结果是这样的:

+ --------- + ----------- +
| lane      | series      |
+ --------- + ----------- +
| 1         | 688         |
| 1         | 685         |
+ --------- + ----------- +

请帮助我使其在 MySQL 5.5 上运行或告诉我为什么结果不同，非常感谢!

更新: 因为我的生产数据库中有大约 300 万条记录，所以我想知道如何快速、尽可能快地获取结果。

Answer 1

试试这个，也许它对你有用。

SELECT lane, series
FROM   tableA
WHERE (
        SELECT count(*)
        FROM   tableA AS f
        WHERE  f.lane = tableA.lane AND 
               f.series >= tableA.series
      ) <= 2
ORDER BY lane, series DESC

SQLFiddle Demo