题解 | 每天登陆最早的用户的内容喜好

SELECT 
    DATE_FORMAT(l1.log_time, '%Y-%m-%d') AS log_day,
    l1.user_id AS user_id,
    u.hobby
FROM login_tb AS l1
LEFT JOIN user_action_tb AS u ON l1.user_id = u.user_id
WHERE 
    l1.log_time IN (
        -- 子查询：找出这一天（不管是谁）的最早登录时间
        SELECT MIN(l2.log_time)
        FROM login_tb AS l2
        WHERE DATE_FORMAT(l2.log_time, '%Y-%m-%d') = DATE_FORMAT(l1.log_time, '%Y-%m-%d')
    )
ORDER BY log_day
#相当于加载了两次login_tb表，在其中一张确定最早登陆时间，另外一张确定user_id