create table tb(a datetime,b decimal(18,2))
insert into tb values('2010-06-27 20:04:00', -125.00)
insert into tb values('2010-06-27 20:12:00', 3000.00)
insert into tb values('2010-06-27 20:33:00', 450.00)
insert into tb values('2010-06-28 19:32:00', -117.00)
insert into tb values('2010-06-28 20:01:00', 3000.00)
insert into tb values('2010-06-28 21:37:00', 450.00)
insert into tb values('2010-06-29 11:33:00', -367.00)
insert into tb values('2010-06-29 21:37:00', -212.00)
insert into tb values('2010-06-30 12:08:00', -590.00)
insert into tb values('2010-06-30 12:49:00', -386.00)
go

select convert(varchar(10),a,120) a , sum(b) b from tb group by convert(varchar(10),a,120)

drop table tb

/*
a          b                                        
---------- ---------------------------------------- 
2010-06-27 3325.00
2010-06-28 3333.00
2010-06-29 -579.00
2010-06-30 -976.00

（所影响的行数为 4 行）

*/

------解决方案--------------------
--生成测试数据：
go
if OBJECT_ID('tbl')is not null
drop table tbl
go
create table tbl(
a datetime,
b int
)
go
insert tbl
select '2010-06-27 20:04:00',-125.00 union all
select '2010-06-27 20:12:00',3000.00union all
select '2010-06-27 20:33:00',450.00union all
select '2010-06-28 19:32:00',-117.00union all
select '2010-06-28 20:01:00',3000.00union all
select '2010-06-28 21:37:00',450.00union all
select '2010-06-29 11:33:00',-367.00union all
select '2010-06-29 21:37:00',-212.00union all
select '2010-06-30 12:08:00',-590.00union all
select '2010-06-30 12:49:00',-386.00

--怎么按每天统计b列的值

select CONVERT(varchar(10),a,120) as a,
SUM(b) as b from tbl  
group by CONVERT(varchar(10),a,120)
/*
a	b
2010-06-27	3325
2010-06-28	3333
2010-06-29	-579
2010-06-30	-976
*/