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(case
				when submit_time is null then 1
        	else 0
			end) as incomplete_cnt,
    	round(sum(case when submit_time is null then 1 else 0 end) /  count(*), 3) as 		complete_rate
	from exam_record
	group by exam_id
) t
where t.incomplete_cnt <> 0