[Programmers] 024 - 년, 월, 성별 별 상품 구매 회원 수 구하기

SELECT YEAR(A.SALES_DATE) YEAR
     , MONTH(A.SALES_DATE) MONTH
     , GENDER
     , COUNT(DISTINCT A.USER_ID) USERS
  FROM ONLINE_SALE A
 INNER JOIN USER_INFO B ON A.USER_ID = B.USER_ID AND B.GENDER IS NOT NULL
 GROUP BY YEAR(A.SALES_DATE), MONTH(A.SALES_DATE), GENDER
 ORDER BY YEAR, MONTH, GENDER