[SQL 문제 풀이] 오프라인/온라인 판매 데이터 통합하기

SELECT 
    DATE_FORMAT(sales_date, '%Y-%m-%d') AS sales_date,
    product_id,
    user_id,
    sales_amount
FROM
    online_sale
WHERE
    MONTH(sales_date) = 3 
UNION 
SELECT 
    DATE_FORMAT(sales_date, '%Y-%m-%d') AS sales_date,
    product_id,
    NULL AS user_id,
    sales_amount
FROM
    offline_sale
WHERE
    MONTH(sales_date) = 3
ORDER BY sales_date ASC , product_id ASC , user_id ASC;