delete from exam_record
where submit_time is null 
or timestampdiff(minute, start_time, submit_time) < 5
order by start_time
limit 3

在得到满足条件后，再去求得前三条就好，这里主要是记住是一个or的条件。

然后求时间的函数 timestampdiff(minute,start_time,submit_time) 可以求得两个timestamp的时间差。