SQL invalid use of group function - mysql

Question

SELECT TIME(ADDTIME(departure, SEC_TO_TIME(SUM(jptl2.run_time)))) AS departure,
service.line_name AS line,
bus.registration
FROM vehicle_journey
LEFT JOIN bus ON bus.journey = vehicle_journey.code AND
ADDTIME(TIME(NOW()), SEC_TO_TIME(3600)) > vehicle_journey.departure AND
ADDTIME(TIME(NOW()), SEC_TO_TIME(3600)) < ADDTIME(vehicle_journey.departure, SEC_TO_TIME(SUM(jptl.run_time)))
INNER JOIN service ON service.reference = vehicle_journey.service_reference
INNER JOIN vehicle_journey_days ON vehicle_journey_days.journey_reference = vehicle_journey.reference
INNER JOIN journey_pattern ON journey_pattern.reference = vehicle_journey.journey_pattern_reference
INNER JOIN journey_pattern_timing_link jptl ON jptl.section = journey_pattern.journey_pattern_section
INNER JOIN journey_pattern_timing_link jptl2 ON jptl2.section = journey_pattern.journey_pattern_section AND jptl2.ID < jptl.id
WHERE jptl.stop = '1900HA030334' AND vehicle_journey_days.day = 'MondayToFriday'
GROUP BY vehicle_journey.id
The above SQL query returns: #1111 - Invalid use of group function
I Know that this issue is caused by the third ON in the LEFT JOIN bus clause. How can I rewrite the query so it saves the same purpose and joins based on the same requirements but without the group function error?
Can I just write the SEC_TO_TIME(SUM(jptl.run_time)) as a sub query?
EG: (SELECT SEC_TO_TIME(SUM(jptl.run_time)))

Answer 1

You cannot use a group function in a [LEFT] JOIN condition, because at this stage, the group are still in the process of being constructed (and the contents of these groups depend on the join conditions, so this would create a sort of circular reference).
First construct your groups, then join the table that depends on these groups:
SELECT groups.*, bus.registration
FROM (
SELECT
TIME(...) AS departure,
service.line_name AS line,
ADDTIME(vehicle_journey.departure, SEC_TO_TIME(SUM(jptl.run_time)))
AS upper_limit,
vehicle_journey.code
AS vj_code,
vehicle_journey.departure
AS vj_departure
FROM vehicle_journey
-- skip "bus" for now
JOIN service ...
-- add all remaining JOIN's here
WHERE ...
GROUP BY vehicle_journey.id
) AS groups
LEFT JOIN bus
ON bus.journey = vj_code
AND ADDTIME(TIME(NOW()), SEC_TO_TIME(3600)) > vj_departure
AND ADDTIME(TIME(NOW()), SEC_TO_TIME(3600)) < upper_limit;
You could also simply move the offending condition down to a HAVING clause, but this would not return records with no match in bus (effectively turning the LEFT JOIN into a INNER JOIN):
SELECT
...
FROM vehicle_journey
LEFT JOIN bus ON bus.journey = vehicle_journey.code AND
ADDTIME(TIME(NOW()), SEC_TO_TIME(3600)) > vehicle_journey.departure
-- no reference to "SUM(jptl.run_time)" here
JOIN service ...
WHERE ...
GROUP BY vehicle_journey.id
-- and here is the reference to "SUM(jptl.run_time)"
HAVING ADDTIME(TIME(NOW()), SEC_TO_TIME(3600))
< ADDTIME(vehicle_journey.departure, SEC_TO_TIME(SUM(jptl.run_time)))