문제
https://leetcode.com/problems/market-analysis-i/
정답
SELECT user_id AS buyer_id, join_date, IFNULL(COUNT(order_date), 0) AS orders_in_2019
FROM Users
LEFT JOIN Orders ON user_id = buyer_id AND YEAR(order_date) = '2019'
GROUP BY user_id
https://leetcode.com/problems/market-analysis-i/
SELECT user_id AS buyer_id, join_date, IFNULL(COUNT(order_date), 0) AS orders_in_2019
FROM Users
LEFT JOIN Orders ON user_id = buyer_id AND YEAR(order_date) = '2019'
GROUP BY user_id