https://leetcode.com/problems/average-time-of-process-per-machine/description/ Easy

Решение

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