明确题意：

计算2021年每个月里试卷作答区用户平均月活跃天数avg_active_days和月度活跃人数mau

问题分解：

• 筛选2021年活跃的用户作答记录：where submit_time is not NULL and YEAR(submit_time)='2021'
• 获取用户-活跃日期的去重结果：select distinct uid, DATE_FORMAT(submit_time, "%Y%m%d") as ymd
• 统计每个月的平均活跃天数和月活人数：
  • 按月份分组：提取月份SUBSTR(ymd, 1, 6) as month；分组group by month
  • 统计月活人数：count(distinct uid) as mau
  • 统计平均活跃天数：总活跃人天数除以活跃人数count(1) / count(distinct uid)

细节问题：

• 表头重命名：as
• 平均活跃天数保留2位小数：round(..., 2)

完整代码：

select SUBSTR(ymd, 1, 6) as `month`,
    round(count(1) / count(distinct uid), 2) as avg_active_days,
    count(distinct uid) as mau
from (
    select distinct uid, DATE_FORMAT(submit_time, "%Y%m%d") as ymd
    from exam_record
    where submit_time is not NULL and YEAR(submit_time)='2021'
) as t_active_day
group by `month`