select exam_id,
(count(start_time) - count(submit_time)) as incomplete_cnt,
round((1- count(submit_time)/ count(start_time)), 3) as complete_rate from
exam_record
group by exam_id
having count(start_time) != count(submit_time)
1、查找有未完成的试卷。 
having count(start_time) != count(submit_time)
2、统计完成的数量，统计总数。计算未完成率