题解 | #统计有未完成状态的试卷的未完成数和未完成率#

这题简单

select 
    exam_id,
    count(start_time) - count(submit_time) incomplete_cnt,#sum(if(score is null, 1, 0)
    round((count(start_time) - count(submit_time))/count(start_time),3)  complete_rate
from 
    exam_record
group by exam_id
having incomplete_cnt >= 1