SELECT '2000-01-01 00:00:00', 223
FROM timeslots
LEFT JOIN timeslotUsers ON timeslots.start = timeslotUsers.start
WHERE timeslots.start = '2000-01-01 00:00:00'
HAVING count(user_id) < timeslots.capacity

SELECT '2000-01-01 00:00:00', 223, timeslots.capacity

INSERT INTO timslotUsers (start, user_id)
SELECT '2000-01-01 00:00:00', 223
FROM timeslots
LEFT JOIN timeslotUsers ON timeslots.start = timeslotUsers.start
WHERE timeslots.start = '2000-01-01 00:00:00'
HAVING count(user_id) < timeslots.capacity