Question

我有如下表格

id   timestamp  speed
1    11:00:01   100
2    11:05:01   110
3    11:10:01   90
4    11:15 :01  80

我需要计算下面的移动平均值

id   timestamp  speed average
1    11:00:01   100   100 
2    11:05:01   110   105
3    11:10:01   90    100
4    11:15:01   80    95

我尝试了什么

SELECT 
*,
(select avg(speed) from tbl t where tbl.timestamp<=t.timestamp) as avg
FROM 
tbl

起初它看起来很容易，但当桌子上的数据膨胀时，它太慢了

任何更快的方法？

Answer 1

您的查询是实现平均运行的一种方法：

SELECT t.*,
       (select avg(speed) from tbl tt where tt.timestamp <= t.timestamp) as avg
FROM tbl t;

另一种方法是使用变量：

select t.*, (sum_speed / cnt) as running_avg_speed
from (select t.*, (@rn := @rn + 1) as cnt, (@s := @s + speed) as sum_speed
      from tbl t cross join
           (select @rn := 0, @s := 0) params
      order by timestamp
     ) t;

tbl(timestamp)上的索引应进一步提高效果。

Answer 2

MySQL是否支持窗口函数？

select
  id, timestamp, speed,
  avg (speed) over (order by timestamp) as average
from tbl

如果它不起作用，虽然我怀疑它是否有效：

select
  min (t1.id) as id, t1.timestamp, min (t1.speed) as speed,
  avg (t2.speed)
from
  tbl t1
  join tbl t2 on
    t2.id <= t1.id
group by
  t1.timestamp
order by
  t1.timestamp

Answer 3

或者在GL的两个答案之间整齐地划分（无论如何都是表现）......

SELECT x.*, AVG(y.speed) avg
  FROM my_table x
  JOIN my_table y
    ON y.id <= x.id
 GROUP 
    BY x.id;

Answer 4

简单的并发解决方案怎么样？

SET @summ=0; SET @counter=0;SELECT *,(@counter := @counter +1) as cnt, (@summ := @summ+speed) as spd, (@summ/@counter) AS avg FROM tbl;

在mysql

4 个答案: