LV4 sql) 상품을 구매한 회원 비율 구하기

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

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