아래 코드에서 오류가 나는데, 어느 부분이 잘못된 건지 질문드립니다.

with table1 as (

select course_id, count(distinct(user_id)) as cnt_checkins from checkins c 

group by course_id

), table2 as (

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

group by course_id

)

select title, a.cnt_checkins, b.cnt_total, 

  (a.cnt_checkins/b.cnt_total) as ratio

from table1 a 

inner join table2 b on a.course_id = b.course_id

inner join courses c2 on a.course_id = c2.course_id