sql日期问题？

Question

create table tb(日期    datetime,数字    int)insert into tb select '2011-08-21 02:10:00.000',10insert into tb select '2011-08-21 02:20:00.000',20insert into tb select '2011-08-21 02:30:00.000',30
insert into tb select '2011-08-22 03:10:00.000',100insert into tb select '2011-08-22 03:20:00.000',200insert into tb select '2011-08-22 03:30:00.000',300drop 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

Answer 1

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

Answer 2

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