* 겪고 있는 문제 상황을 최대한 자세하게 작성해주세요.

* 문제 해결을 위해 어떤 시도를 해보았는지 구체적으로 함께 알려주세요.

SELECT a.course_id,

a.cnt_checkins,

b.cnt_total,

(a.cnt_checkins/b.cnt_total) as ratio

from

(

select course_id, count(DISTINCT (user_id)) as cnt_checkins from checkins

group by course_id

) a

inner join

(

select course_id,count(*) as cnt_total, from orders

group by course_id

) b

on a.course_id = b.course_id

inner join courses c on a.course_id = c.course_id