题解 | #平均活跃天数和月活人数#

SELECT DATE_FORMAT(submit_time, "%Y%m") AS month,
        ROUND(COUNT(DISTINCT uid, DATE_FORMAT(submit_time, '%y%m%d'))/COUNT(DISTINCT uid), 2) AS avg_active_days,
        COUNT(DISTINCT uid) AS mau
FROM exam_record
WHERE submit_time is not null AND YEAR(submit_time) = "2021"
GROUP BY DATE_FORMAT(submit_time, "%Y%m")

以上为正确代码

以下为错误代码，请问 为什么不能使用GROUP BY MONTH(start_time)，并且即使修改第六行为GROUP BY DATE_FORMAT(submit_time, "%Y%m")，第一行不进行修改，结果也仍然报错，求教求教

SELECT CONCAT(YEAR(start_time), MONTH(start_time)) AS month,
        COUNT(uid)/COUNT(DISTINCT uid) AS avg_active_days,
        COUNT(DISTINCT uid) AS mau
FROM exam_record
WHERE submit_time is not null AND YEAR(start_time) = "2021"
GROUP BY MONTH(start_time)