[Programmers] 상품을 구매한 회원 비율 구하기

WITH J2021 AS
(  
    -- 2021년 가입한 회원
    SELECT USER_ID
      FROM USER_INFO 
     WHERE EXTRACT(YEAR FROM JOINED) = '2021'
),
B2022 AS
(   
     -- 2022년 구매한 회원
    SELECT USER_ID
         , EXTRACT(YEAR  FROM SALES_DATE)  AS YEAR
         , EXTRACT(MONTH FROM SALES_DATE)  AS MONTH
      FROM ONLINE_SALE
     WHERE EXTRACT(YEAR FROM SALES_DATE) = '2022'
)
SELECT B.YEAR
     , B.MONTH
     , COUNT(DISTINCT B.USER_ID) AS PURCHASED_USERS
     , ROUND(COUNT(DISTINCT B.USER_ID) / (SELECT COUNT(1) FROM J2021), 1) AS PUCHASED_RATIO
  FROM J2021 A
  JOIN B2022 B
    ON A.USER_ID = B.USER_ID
 GROUP BY B.YEAR, B.MONTH
 ORDER BY B.YEAR, B.MONTH