프로그래머스 - 상품을 구매한 회원 비율 구하기

SELECT YEAR(S.SALES_DATE) AS YEAR, MONTH(S.SALES_DATE) AS MONTH, COUNT(DISTINCT(S.USER_ID)) AS PURCHASED_USERS, ROUND(COUNT(DISTINCT(S.USER_ID))/(SELECT COUNT(*) FROM USER_INFO WHERE YEAR(JOINED) = 2021),1) AS PURCHASED_RATIO
FROM USER_INFO I JOIN ONLINE_SALE S ON I.USER_ID = S.USER_ID
WHERE YEAR(I.JOINED) = 2021 
GROUP BY YEAR(S.SALES_DATE),MONTH(S.SALES_DATE)
ORDER BY YEAR(S.SALES_DATE), MONTH(S.SALES_DATE);