题解 | #统计作答次数#
统计作答次数
http://www.nowcoder.com/practice/45a87639110841b6950ef6a12d20175f
select sum(case when start_time is not null then 1 end) ,sum(case when submit_time is not null and score is not null then 1 end) ,count(distinct case when submit_time is not null and score is not null then exam_id end) from exam_record
