题解 | #月均完成试卷数不小于3的用户爱作答的类别#

这道题的解题思路是 先找出exam_record表月均完成试卷数不小于3的用户-带入原表取符合条件的用户数据-再根据两个表的嵌套-带入examination_info的取数-实现按tag的取数
1、第一部分关键点在于uid的取数，两重分组要注意重复值的剔除
2、第二部分要做好两表之前的链接

SELECT tag,
count(*) tag_cnt
FROM examination_info ei
join exam_record
USING(exam_id)
JOIN (
    select distinct uid
    from exam_record 
    where submit_time IS NOT NULL
    group by uid, month(submit_time)
    having COUNT(submit_time)>=3 ) er
USING(uid)
group by tag
order by tag_cnt desc