running average in mysql

Question

I have the table like below

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

I need to calcu

Answer 1

Your query is one way to do a running average:

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

The alternative is to use variables:

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;

An index on tbl(timestamp) should further improve performance.