**判空 用is null 不是 = null **
update exam_record set submit_time = '2099-01-01 00:00:00', score = 0
where start_time < '2021-09-01 00:00:00' and submit_time is null;
**判空 用is null 不是 = null **
update exam_record set submit_time = '2099-01-01 00:00:00', score = 0
where start_time < '2021-09-01 00:00:00' and submit_time is null;