题解 | #月均完成试卷数不小于3的用户爱作答的类别#
月均完成试卷数不小于3的用户爱作答的类别
http://www.nowcoder.com/practice/b1d13efcfa0c4ecea517afbdb9090845
select
e2.tag,
count(*) cnt
from exam_record e1 left join examination_info e2 on e1.exam_id=e2.exam_id
WHERE
e1.uid in (
select uid from exam_record
WHERE
submit_time is not NULL
group by uid ,date_format(start_time,'%Y%m')
having count(*)>=3
)
group by e2.tag
order by cnt desc
其实，了解题意 就好做了，是求用户ID完成作答大于3情况下，查看符合条件用户ID，试卷类别，这些试卷类中有几次作答
