题解 | #统计有未完成状态的试卷的未完成数和未完成率#

select * from (
select exam_id,
sum(if(submit_time is null,1,0)) as incomplete_cnt
,round(sum(if(submit_time is null,1,0))/count(*),3) as incomplete_rate 
from exam_record group by exam_id ) t1 where t1.incomplete_cnt != 0