1661. Average Time of Process per Machine

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.ma...