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https://leetcode.com/problems/market-analysis-i/ [LEFT JOIN 사용] SELECT Users.user_id as 'buyer_id' ,Users.join_date ,IFNULL(COUNT(order_date), 0) AS 'orders_in_2019' FROM Users LEFT JOIN Orders ON Orders.buyer_id = Users.user_id AND YEAR(Orders.order_date) = '2019' GROUP BY Users.user_id [CTE 사용] WITH O AS( SELECT buyer_id, COUNT(*) AS cnt FROM Orders WHERE YEAR(order_date) = '2019' GROUP BY buy..