题解 | #月总刷题数和日均刷题数#

### 解题思路：计算每月的题目练习数以及每月的日均题目练习率，最后使用union 或者 union all 连接数据表的时候，   order by 子句应放在最后面。

select  date_format(submit_time,'%Y%m') submit_month ,
        count(submit_time) month_q_cnt,
        round(count(submit_time)/day(last_day(max(submit_time))),3) avg_day_q_cnt 
  from practice_record 
  where year(submit_time) = 2021 
  group by date_format(submit_time,'%Y%m') 
union 
  select '2021汇总' submit_month,
         count(1) month_q_cnt ,
         round(count(1)/31,3) avg_day_q_cnt 
    from practice_record 
   where year(submit_time) = 2021 

   order by submit_month ;