[SQL]프로그래머스 Lv.4 상품을 구매한 회원 비율 구하기

✅ 코드

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

SELECT문 서브쿼리와 INNER JOIN을 사용한 풀이입니다.