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, incomplete_cnt,
    ROUND(incomplete_cnt / record_cnt, 3) AS incomplete_rate
FROM (
    SELECT exam_id, 
        COUNT(start_time) AS record_cnt,
        COUNT(IF(submit_time IS NULL, exam_id, NULL)) AS incomplete_cnt
    FROM exam_record
    GROUP BY exam_id
)record_table
WHERE incomplete_cnt > 0