Question

这是我的表sampletable4（只是一个示例）。我的表是数百万的数据。：

      app_id    latency    -- app_id is char(255), latency is float
    | 1111   |    0.940056 |
    | 1110   |    0.261509 |
    | 3211   |   0.0506242 |
    | 3210   |    0.468594 |
    | 3110   |    0.191095 |

其中app_id是一个字符串，延迟是浮点数。延迟衡量API调用每个应用程序的速度。

我想为每个app_id打印出最慢（或最高延迟）。我的查询不限制为最慢的5％。我想将查询限制为每个app_id的最慢5％。

这是我的问题：

  SELECT 
  latency  
  FROM sampleTable4
  WHERE app_id = '5697'
  ORDER BY latency DESC;

这是app_id ='5697'的特定情况。我如何更改它以便打印出5％的延迟？

Answer 1

SELECT latency FROM  (SELECT latency,(SELECT FLOOR(COUNT(*)*0.05) AS per_count   
  FROM sampleTable4
  WHERE app_id = '5697'  ) AS 5per_count,IF(@cnt=NULL,@cnt:=1,@cnt:=@cnt+1) AS cnt 
  FROM sampleTable4 ,(SELECT @cnt:=0) AS T
  WHERE app_id = '5697'
  ORDER BY latency DESC ) FT       
  WHERE cnt<=5per_count

MySql：如何限制打印的行数

1 个答案: