select exam_id, sum(if (submit_time is null, 1, 0)) as 	incomplete_cnt,
round(sum(if (submit_time is null, 1, 0))/count(start_time),3) as complete_rate
from exam_record
group by exam_id 
having count(start_time) > count(submit_time)

首先是对exam_id 进行group by,然后未完成就要满足

count(start_time) > count(submit_time)