题解 | #统计作答次数#

关键在于统计`complete_exam_cnt`

思路：

• 用case 语句进行筛选，输出那些已完成提交的exam_id。（用if语句也可），而其他情况记为null
• 用distinct进行对exam_id去重
• 用count()进行计数。count(col)不会统计null的行

select
    count(start_time) as total_pv,
    count(submit_time) as complete_pv,
        count(
        distinct (
            case
                when submit_time is not null then exam_id
                else null
            end
        )
    ) as complete_exam_cnt
from
    exam_record