강사님과 똑같이 한거같은데 오류가 뜹니다 어디가 문제인걸까요

작성한 코드 및 에러 메세지

SELECT a.course.id, a.cnt_chekcins, b.cnt_total,(a.cnt_chekcins/b.cnt_total) as ratio,

c.* 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 o 

group by course_id 

) b on a.course_id = b.course_id

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