4-4 with절 연습 부분에서 계속 오류가나서 코드 스니펫 복붙해봤는데도 결과값이 안나와요

작성한 코드 및 에러 메세지

with table1 as (

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

group by course_id

), table2 as (

select course_id, count(*) as cnt_total from orders

group by course_id

)

select c.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 c on a.course_id = c.course_id