average_cohort_total_assistance_duration.sql

SELECT avg(total_duration) AS average_total_duration 
FROM (
  SELECT cohorts.name AS cohort,
    sum(completed_at - started_at) AS total_duration
  FROM assistance_requests
  JOIN students ON student_id = students.id
  JOIN cohorts ON cohort_id = cohorts.id
  GROUP BY cohort
  ORDER BY total_duration
) AS r;