slot_availability

CREATE VIEW slot_availability AS
SELECT
  s.slot_id,
  s.exhibition_id,
  e.title,
  s.slot_date,
  s.start_time,
  s.capacity,
  count(b.booking_id)              AS booked,
  count(h.scan_id)                 AS admitted,
  s.capacity - count(b.booking_id) AS remaining
FROM exhibition_slot s
JOIN exhibition e ON e.exhibition_id = s.exhibition_id
LEFT JOIN exhibition_booking b ON b.slot_id = s.slot_id
LEFT JOIN hall_napoleon_scan h ON h.booking_id = b.booking_id
GROUP BY s.slot_id;