3주차 숙제를 풀고나서 답안을 확인해보았는데

'enrolled_id별'  이래서 enrolled_id 만 추가했는데

group by 에 e.user_id 는 왜 들어가는것일까요?

작성한 코드 및 에러 메세지

'제가 작성한 코드'
SELECT e.enrolled_id,
	   e.user_id,
	   count(*) as 'cnt'
	from enrolleds e
inner join enrolleds_detail ed on e.enrolled_id = ed.enrolled_id
where ed.done = '1'
group by e.enrolled_id
order by COUNT(*) desc 

'답안코드'
select  e.enrolled_id,
	     e.user_id,
	     count(*) as cnt
  from enrolleds e
 inner join enrolleds_detail ed on e.enrolled_id = ed.enrolled_id
 where ed.done = 1
 group by e.enrolled_id, e.user_id
 order by cnt desc