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

sum(if(score is null,1,0))的用法