sql日期问题？

create table tb

(

日期 datetime,

数字 int

)

insert into tb select '2011-08-21 02:10:00.000',10

insert into tb select '2011-08-21 02:20:00.000',20

insert into tb select '2011-08-21 02:30:00.000',30


insert into tb select '2011-08-22 03:10:00.000',100

insert into tb select '2011-08-22 03:20:00.000',200

insert into tb select '2011-08-22 03:30:00.000',300



drop table tb

----------------------------
--我想求2011-8-21这一天的数字这一列的总和


select sum(数字)from tb where 日期='2011-8-21'

但是结果却是null

------解决方案--------------------

SQL code

select sum(数字)from tb where 日期 between '2011-8-21 00:00:00' and '2011-8-21 23:59:59'

------解决方案--------------------
select sum(num) from tablename where convert(vcrchar(10),日期,120)='2011-08-21'