JOIN - 상품을 구매한 회원 비율 구하기

https://school.programmers.co.kr/learn/courses/30/lessons/131534

프로그래머스

 

select DATE_FORMAT(SALES_DATE,'%Y') as YEAR,DATE_FORMAT(SALES_DATE,'%m') AS MONTH
,count(distinct S.user_ID) as PUCHASED_USERS
, round((count(distinct S.user_ID)/
                   (select 
                    COUNT(USER_ID) 
                          from USER_INFO 
                    WHERE JOINED LIKE '2021-%')),1) as PUCHASED_RATIO
from USER_INFO AS I
join ONLINE_SALE AS S on I.USER_ID =S.USER_ID
where I.USER_ID IN (SELECT I.USER_ID
from USER_INFO AS I
join ONLINE_SALE AS S on I.USER_ID =S.USER_ID
where  DATE_FORMAT(JOINED,'%Y') ='2021')
group by  DATE_FORMAT(SALES_DATE,'%Y'),DATE_FORMAT(SALES_DATE,'%m')