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 (submit_time is null, 1, 0)) incomplete_cnt,
    ROUND(SUM(if (submit_time is null, 1, 0)) / count(*), 3) complete_rate
from
    exam_record
group by
    exam_id
HAVING
    incomplete_cnt > 0