interval 时间间隔关键字，常和date_add() 或 date_sub()搭配使用。

insert into exam_record (uid, exam_id, start_time, submit_time, score) 
values
(1001, 9001, "2021-09-01 22:11:12", "2021-09-01 22:11:12" + INTERVAL 50 minute, 90),
(1002, 9002, "2021-09-04 07:01:02",NULL,NULL);

or

insert into exam_record (uid, exam_id, start_time, submit_time, score) 
values
(1001, 9001, "2021-09-01 22:11:12", DATE_ADD("2021-09-01 22:11:12", INTERVAL 50 minute), 90),
(1002, 9002, "2021-09-04 07:01:02",NULL,NULL);