题解 | 月均完成试卷数不小于3的用户爱作答的类别

select
    tag,
    count(tag) tag_cnt
from
    test.exam_record er
    join test.examination_info ei on er.exam_id = ei.exam_id
where
    uid in (
        select
            uid
        from
            test.exam_record er
            join test.examination_info ei on er.exam_id = ei.exam_id
        where
            submit_time between start_time and last_day  (er.start_time)
            and month (start_time) = 9
        group by
            uid
        having
            sum(
                case
                    when submit_time is not null then 1
                    else 0
                end
            ) >= 3
    )
group by
    tag
order by
    tag_cnt desc