如何用sql拉取累计数

select a.time as 年月,sum(b.num) as 累计注册数 from (  select DATE_FORMAT(addTime,'%Y年%m月') as time ,count(1) as num from user u  where addtime > '2017-01-01'  group by DATE_FORMAT(addTime,'%Y年%m月')) a join(  select DATE_FORMAT(addTime,'%Y年%m月') as time ,count(1) as num from user u  where addtime > '2017-01-01'  group by DATE_FORMAT(addTime,'%Y年%m月')) b on a.time >= b.time group by a.time;