求合并相邻记录的语句

表记录如下：
  NAME QUANTITY
----------------------------
a 1
a 1
b 2
b 2
a 1
a 1
b 2
b 2
要求相邻记录name相同的字段合并数量，结果如下：
  NAME QUANTITY
----------------------------
a 2
b 4
a 2
b 4
sql语句如何写？

SQL code

if object_id('[tb]') is not null drop table [tb]
go
create table [tb]([NAME] varchar(1),[QUANTITY] int)
insert [tb]
select 'a',1 union all
select 'a',1 union all
select 'b',2 union all
select 'b',2 union all
select 'a',1 union all
select 'a',1 union all
select 'b',2 union all
select 'b',2

;with t1 as
(select rn=row_number() over(order by getdate()),* from [tb]),
t2 as
(select rn-(select count(1) from t1 where name=t.name and rn<=t.rn) gid,* from t1 t)

SELECT NAME,SUM(QUANTITY) AS QUANTITY
FROM T2
GROUP BY NAME,GID
ORDER BY MIN(RN)

/**
NAME QUANTITY
---- -----------
a    2
b    4
a    2
b    4

(4 行受影响)
**/

--tb为原来的表，tmp为加了序号的临时表
select id = identity(int,1,1) into tmp from tb

select name , sum(QUANTITY) QUANTITY from tmp group by name , (px-1) / 2

--tb为原来的表，tmp为加了序号的临时表
select id = identity(int,1,1) into tmp from tb

select name , sum(QUANTITY) QUANTITY from tmp group by name , (id-1) / 2