[프로그래머스/4단계/MYSQL] 오프라인/온라인 판매 데이터 통합하기

https://school.programmers.co.kr/learn/courses/30/lessons/131537

프로그래머스

SELECT DATE_FORMAT(SALES_DATE,"%Y-%m-%d") SALES_DATE,
    PRODUCT_ID,
    USER_ID,
    SALES_AMOUNT
FROM ONLINE_SALE
WHERE month(SALES_DATE)='3'

UNION ALL 

SELECT DATE_FORMAT(SALES_DATE,"%Y-%m-%d") SALES_DATE,
    PRODUCT_ID,
    NULL AS USER_ID,
    SALES_AMOUNT
FROM OFFLINE_SALE
WHERE month(SALES_DATE)='3'

ORDER BY SALES_DATE , PRODUCT_ID , USER_ID;