# [1661. Average Time of Process per Machine](https://leetcode.com/problems/average-time-of-process-per-machine/description/?envType=study-plan-v2&envId=top-sql-50)
這一題要求我們找出每一個時間差
所以我們取道小數點第三位的同時 平均我們的值
將自己self join 將a1設為start a2設為end

SELECT a1.machine_id , ROUND(AVG(a2.timestamp - a1.timestamp),3) AS processing_time
FROM Activity a1 JOIN Activity a2
ON a1.machine_id = a2.machine_id AND a1.process_id = a2.process_id 
AND a1.activity_type = 'start' AND a2.activity_type = 'end'
GROUP BY a1.machine_id