Mysql 누적 집계

-- 누적 변수 선언 
SET @va = 0;

SELECT a.order_date,
       a.sum_amount,
       (@va := @va + a.sum_amount) as cumsum_amount -- 누적 값
FROM ( SELECT date_format(order_time, '%Y-%m') order_date, 
      sum(order_amount) sum_amount
FROM Test_table GROUP BY order_date ORDER BY order_date ) a;