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(*) AS tag_cnt
FROM exam_record t1
LEFT JOIN examination_info t2
ON t1.exam_id = t2.exam_id
WHERE uid IN (
    SELECT uid
    FROM exam_record
    WHERE submit_time IS NOT NULL
    GROUP BY DATE_FORMAT(submit_time, "%Y%m"), uid
    HAVING COUNT(*) >= 3
)
GROUP BY tag
ORDER BY tag_cnt DESC;

试了两次，运行成功了，我自己都没想到，哈哈！先查有效的用户(当月完成次数大约3)，这个当月比较讨厌，所以要先根据年月分组，再根据uid分组，才能计算出来；然后就是联表查询了