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(r.exam_id) as tag_cnt  # 只需作答即可，所以count(exam_id)
FROM
exam_record r
LEFT JOIN examination_info i
ON r.exam_id=i.exam_id
WHERE uid in                    # 对用户进行过滤，满足月均完成不小于3
    (SELECT uid FROM exam_record r
     WHERE
          month (submit_time) = 09
     GROUP by
           uid
     HAVING
           count(score)>=3) 
GROUP by tag
ORDER BY tag_cnt desc