题解 | #统计有未完成状态的试卷的未完成数和未完成率#
统计有未完成状态的试卷的未完成数和未完成率
http://www.nowcoder.com/practice/69fc2b1df4144c0991e4b8280d8aba27
# 这里其实可以有很多不同的方法, 比如sum(case when submit_time is null then 1 else 0)
# sum(if(submit_time is null, 1, 0)) 都可以. 有没有大佬知道这些方法哪个效率更高?
select exam_id,
count(start_time) - count(submit_time),
round((count(start_time) - count(submit_time)) / count(1), 3)
from exam_record
group by exam_id
having count(start_time) - count(submit_time) > 0
