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在mysql中实现如上数据累加效果，可使用set，定义用户变量。
sql 语句：
SET @x = 0;
SELECT
t1.months,
t1.register_num ,
@x := @x+t1.register_num AS total
FROM
(
SELECT
DATE_FORMAT(createTime, "%Y%m") months,
count(id) AS register_num
FROM
t_user
GROUP BY
months asc
) t1
