题解 | #21年8月份练题总数#

首先，我们需要先对21年8月份进行表的筛选，代码如下：

select * from question_practice_detail 
where month(date)=8 and year(date)=2021

接着，我们需要从上述21年8月份的表中，按device_id分组，统计总用户数和练习过题目的总次数，代码如下：

select
	count(distinct device_id) as x,
	count(question_id) as y
from
	(select * from question_practice_detail
	where month(date)=8 and year(date)=2021) as a
group by device_id

其中，group by device_id按device_id进行分组，count函数对每个device_id分组进行计数，distinct表示统计唯一的device_id字段。 最后，我们可以对上表进行一个求和统计，代码如下：

select
    sum(b.x) as did_cnt,
    sum(b.y) as question_cnt
from
    (select
        count(distinct device_id) as x,
        count(question_id) as y
    from
        (select
            *
        from 
            question_practice_detail
        where 
            month(date)=8 and year(date)=2021) as a
    group by device_id) as b;