풀이 코드

SELECT DATE_FORMAT(SALES_DATE, "%Y-%m-%d") AS SALES_DATE,
PRODUCT_ID, USER_ID, SALES_AMOUNT
FROM ONLINE_SALE
WHERE SALES_DATE LIKE "2022-03%"
UNION ALL
SELECT DATE_FORMAT(SALES_DATE, "%Y-%m-%d") AS SALES_DATE,
PRODUCT_ID, 
NULL AS USER_ID
, SALES_AMOUNT
FROM OFFLINE_SALE
WHERE SALES_DATE LIKE "2022-03%"
ORDER BY SALES_DATE, PRODUCT_ID, USER_ID