[Programmers] 017 - 상품을 구매한 회원 비율 구하기

(예시는 길어서 생략)

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