[프로그래머스] 오프라인/온라인 판매 데이터 통합하기

SELECT A.*
FROM 
(
SELECT SUBSTRING(SALES_DATE,1,10)
        , PRODUCT_ID
        , USER_ID
        , SUM(SALES_AMOUNT)
FROM ONLINE_SALE 
WHERE SUBSTRING(SALES_DATE,6,2) = '03'
GROUP BY 1,2,3

UNION ALL 

SELECT SUBSTRING(OFF.SALES_DATE,1,10)
        , OFF.PRODUCT_ID
        , NULL AS USER_ID
        , SUM(OFF.SALES_AMOUNT)
FROM OFFLINE_SALE OFF
WHERE SUBSTRING(OFF.SALES_DATE,6,2) = '03'
GROUP BY 1,2,3
) A
ORDER BY 1,2,3