题解 | #统计有未完成状态的试卷的未完成数和未完成率#

select exam_id,
sum(if(submit_time is null,1,0)) 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 incomplete_cnt != 0