sum()over()计算累加和函数

SELECT exam_id, start_month, month_cnt,
    SUM(month_cnt)OVER(PARTITION BY exam_id ORDER BY start_month)
FROM (
    SELECT exam_id,
        DATE_FORMAT(start_time, '%Y%m') AS start_month,
        COUNT(start_time) AS month_cnt
    FROM exam_record
    GROUP BY exam_id, start_month
)sel_t