题解 | 月均完成试卷数不小于3的用户爱作答的类别

select i.tag,count(i.tag) as tag_cnt 
from exam_record as r inner join examination_info as i on r.exam_id=i.exam_id 
where r.uid in #返回单列
(
    select
        uid#只能筛选uid列
    from
        exam_record
    where
        score is not null
        and start_time like "2021-09%"
        and submit_time like "2021-09%"
    group by
        uid
    having
        count(score) >= 3
) 
group by i.tag
order by tag_cnt DESC