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(exam_id) - count(submit_time) as incomplete_cnt,
round((count(exam_id) - count(submit_time)) / count(exam_id), 3) as incomplete_rate
from exam_record
where exam_id in (select exam_id
                  from exam_record
                  where submit_time IS NULL
                 )
group by exam_id;