题解 | #平均活跃天数和月活人数#

distinct函数作用于多列 count是不能统计多个字段的，下面的SQL在SQL Server和Access中都无法运行。

select count(distinct name, id) from A; 

若想使用，请使用嵌套查询，如下：

select count(*) from (select distinct xing, name from B) AS M;

参考资料： https://www.cnblogs.com/kelly1314/p/10830136.html

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');