1661-average-time-of-process-per-machine.sql

# Write your MySQL query statement below
SELECT 
    a1.machine_id,
    ROUND(AVG(a2.timestamp - a1.timestamp), 3) AS processing_time
FROM 
    Activity a1,
    Activity a2
WHERE 
    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 
    machine_id;