--1.统计月均完成数大于3的用户
WITH t AS (
    SELECT
        er.*,
        ei.tag,
        count( score ) over ( PARTITION BY uid, DATE_FORMAT( submit_time, '%Y%m' ) ) AS score_num 
    FROM
        exam_record er
        JOIN examination_info ei ON er.exam_id = ei.exam_id 
    ) 
  
--2.统计这些用户完成试卷的类别及相应次数，次数包括未完成
SELECT
    tag,
    count( start_time ) tag_cnt 
FROM
    t 
WHERE
    uid IN ( SELECT uid FROM t WHERE score_num >= 3 ) 
GROUP BY
    exam_id 
ORDER BY
    tag_cnt DESC