题解 | 牛客直播开始时各直播间在线人数
牛客直播开始时各直播间在线人数
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SELECT at.course_id, ct.course_name, SUM(IF(TIME(at.in_datetime) <= '19:00:00', 1, 0)) AS online_num FROM attend_tb AS at LEFT OUTER JOIN course_tb AS ct ON at.course_id = ct.course_id GROUP BY at.course_id, ct.course_name ORDER BY at.course_id;