SQL累加问题，根据日期降序累加

create table User_Salary (UserName nvarchar(200), Month nvarchar(20), Salary int)
go
insert into User_Salary (UserName,Month,Salary ) values('AAA','2010/12',1000)
insert into User_Salary (UserName,Month,Salary ) values('AAA','2011/01',2000)

insert into User_Salary (UserName,Month,Salary ) values('AAA','2011/02',3000)

insert into User_Salary (UserName,Month,Salary ) values('BBB','2010/12',2000)

insert into User_Salary (UserName,Month,Salary ) values('BBB','2011/01',2500)

insert into User_Salary (UserName,Month,Salary ) values('BBB','2011/02',2500)


上面是原表格，想实现成（UserName相同，根据日期降序进行汇总，即汇总日期小于此行日期且UserName相同所有数据），如下图

该如何写SQL语句

select u.*
,(select SUM(salary) from User_Salary where User_Salary.Month<=u.month and User_Salary.UserName = u.userName) as 总和
from User_Salary u

select u.*
,(select SUM(salary) from User_Salary where User_Salary.Month>=u.month and User_Salary.UserName = u.userName) as 总和
from User_Salary u

这个才是按照降序来的