题解 | #删除记录（二）#
删除记录（二）
http://www.nowcoder.com/practice/964c9f7fffbb4ab18b507cfed4111b4a
**空或者时间小于5 用到了 is null, timestampdiff **
delete from exam_record
where submit_time is null or timestampdiff(minute, start_time, submit_time) < 5
order by start_time
limit 3
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