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,
count(case when score is null then 1 else null end) incomplete_cnt,
format(count(case when score is null then 1 else null end)/count(id),3) incomplete_rate
from exam_record
group by exam_id
having incomplete_cnt>0