union

union - 필드를 붙여줄 때 사용

(
	select '7월' as month, c.title, c2.week, count(*) as cnt from checkins c2
	inner join courses c on c2.course_id = c.course_id
	inner join orders o on o.user_id = c2.user_id
	where o.created_at < '2020-08-01'
	group by c2.course_id, c2.week
  order by c2.course_id, c2.week
)
union all
(
	select '8월' as month, c.title, c2.week, count(*) as cnt from checkins c2
	inner join courses c on c2.course_id = c.course_id
	inner join orders o on o.user_id = c2.user_id
	where o.created_at >= '2020-08-01'
	group by c2.course_id, c2.week
  order by c2.course_id, c2.week
)

union 을 사용하면 내부 정렬이 깨지는데 서브쿼리를 사용하면 됨. 4주차에 배울 예정