**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
**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