SELECT
    SEC_TO_TIME(SUM(TIME_TO_SEC(TIME(end_time)) - TIME_TO_SEC(TIME(start_time)))), user_id, DATE(start_time)
FROM
(SELECT
    start_time, CASE WHEN TIME(end_time) > '09:00:00' THEN DATE_ADD(DATE(end_time), INTERVAL 9 HOUR) ELSE end_time END AS end_time, user_id
FROM
    meetings
WHERE
    TIME(start_time) < '09:00:00'

UNION

SELECT
    CASE WHEN TIME(start_time) < '17:00:00' THEN DATE_ADD(DATE(start_time), INTERVAL 17 HOUR) ELSE start_time END AS start_time, end_time, user_id
FROM
    meetings
WHERE
    TIME(end_time) > '17:00:00') AS clamped_times
GROUP BY user_id, DATE(start_time)