MySQL throws error 1111 when I try to run my query, how do I fix my group by and having clauses?

Question

I am running a query to find which service has been scheduled the least often and keep running into error 1111: invalid use of group function. I have tried reformatting my query a few times and cannot resolve the issue. Is there a different syntax I should use?

The service table contains service_id, service_description, and service_cost and the scheduled_service table contains scheduled_service_id, appointment_id, and service_id. I have tried rewording my query a few times and all give me the same error.

This is my most recent attempt.

SELECT s.service_id, COUNT(ss.scheduled_service_id) AS ' # Scheduled' 
FROM service s JOIN scheduled_service ss
ON s.service_id = ss.service_id
GROUP BY service_id
HAVING COUNT(ss.scheduled_service_id) =
(SELECT MIN(COUNT(scheduled_service_id)) FROM scheduled_service GROUP BY service_id);

I can get the count for all services performed using this query but trying to alter it to giving me just the minimum is what I cannot do.

SELECT service_id, COUNT(scheduled_service_id) AS '# Scheduled' 
FROM scheduled_service 
GROUP BY service_id ;

I am expecting to get the service that has been scheduled the least amount of times.

Answer 1

You can ORDER BY the count and then LIMIT the number of returned rows to 1.

SELECT s.service_id, COUNT(ss.scheduled_service_id) AS ' # Scheduled' 
FROM service s JOIN scheduled_service ss
ON s.service_id = ss.service_id
GROUP BY service_id
ORDER BY COUNT(ss.scheduled_service_id)
LIMIT 1

Answer 2

If you want the least often, you can use:

SELECT s.service_id, 
       COUNT(ss.scheduled_service_id) as num_scheduled
FROM service s JOIN
     scheduled_service ss
     ON s.service_id = ss.service_id
GROUP BY s.service_id
ORDER BY num_scheduled ASC
LIMIT 1;

This query doesn't actually require the JOIN , so you can do:

SELECT ss.service_id, 
       COUNT(ss.scheduled_service_id) as num_scheduled
FROM scheduled_service ss
GROUP BY ss.service_id
ORDER BY num_scheduled ASC
LIMIT 1;

Unless you want to count services that have been scheduled zero times. In that case, you want a LEFT JOIN :

If you want the least often, you can use:

SELECT s.service_id, 
       COUNT(ss.scheduled_service_id) as num_scheduled
FROM service s LEFT JOIN
     scheduled_service ss
     ON s.service_id = ss.service_id
GROUP BY s.service_id
ORDER BY num_scheduled ASC
LIMIT 1;