select c.title, c2.week, count(*) as cnt from courses c 

inner join checkins c2 on c.course_id = c2.course_id 

inner join orders o on c2.course_id = o.course_id  

where o.created_at >= '2020-08-01'

group by c.title, c2.week 

order by c.title, c2.week

두번째 join 할 때 첫번째 join에 쓴 필드명을 쓰면 오류가 생기나요..?