delete from exam_record
where (start_time is not null and submit_time is null) or timestampdiff(minute, start_time, submit_time) < 5
order by start_time asc
limit 3;

删除delete from 也可以用order by

limit 两种方式：

1. limit N 提取前N行记录

2.limit N, M（分页） 从N+1开始显示M个数据。

妙用：提取第Kth大(小)的值 可以运用 limit Kth-1，1