题解 | #统计每个学校的答过题的用户的平均答题数#
删除记录（一）
http://www.nowcoder.com/practice/d331359c5ca04a3b87f06b97da42159c
DELETE FROM exam_record
WHERE TIMESTAMPDIFF(second,start_time,submit_time) <300
AND score <60
开始用minute(submit_time) - minute(start_time)发现有漏的，于是用timestamp了
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