SELECT
  IFNULL(s.film_id, 'Итого') id,
  COUNT(*) AS total_all_time,
  COUNT(*) / COUNT(DISTINCT b.seans_id) AS avg_buyed,
  SUM(s.price) AS total_price
FROM seans s
JOIN bilet b ON s.id = b.seans_id
GROUP BY s.film_id WITH ROLLUP