分级求和

a                b
20061105           10
20061106           12
20061107           13
20061108           14
20061109           15

表是这样的格式
算出每个日期之前的所有的b的值，所得结果如下

   a                b
20061105           10
20061106           22
20061107           35
20061108           49
20061109           64

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创建表：create table test (a char(8),b int)
插入数据：insert into test4 values ('20061105',10)
          insert into test4 values ('20061106',12)
          insert into test4 values ('20061107',13)
          insert into test4 values ('20061108',14)
          insert into test4 values ('20061109',15)
sql语句：select a,b=(select sum(b) from test where a<=t.a) from test as t