create table testDate
(
 ID int primary key identity(1,1),
 DateLoan datetime
)
insert into testDate values('2012-4-27 07:20:10')
insert into testDate values('2012-4-28 07:20:10')
insert into testDate values('2012-4-28 08:20:10')
insert into testDate values('2012-4-28 08:30:10')
insert into testDate values('2012-4-28 08:40:10')
insert into testDate values('2012-4-28 08:50:10')
insert into testDate values('2012-4-28 09:00:00')
insert into testDate values('2012-4-28 09:20:10')
insert into testDate values('2012-4-28 09:20:10')

SELECT * FROM testDate
/*
ID          DateLoan
----------- -----------------------
1           2012-04-28 07:20:10.000
2           2012-04-28 08:20:10.000
3           2012-04-28 08:30:10.000
4           2012-04-28 08:40:10.000
5           2012-04-28 08:50:10.000
6           2012-04-28 09:00:00.000
7           2012-04-28 09:20:10.000
8           2012-04-28 09:20:10.000
9           2012-04-27 07:20:10.000

(9 行受影响)
*/
SELECT * FROM testDate WHERE SUBSTRING(CONVERT(NVARCHAR(20),DateLoan,120),1,10) between '2012-04-27' and '2012-04-28' and SUBSTRING(CONVERT(NVARCHAR(20),DateLoan,114),1,8) between '08:00:00' and '09:00:00' 
/*
ID          DateLoan
----------- -----------------------
2           2012-04-28 08:20:10.000
3           2012-04-28 08:30:10.000
4           2012-04-28 08:40:10.000
5           2012-04-28 08:50:10.000
6           2012-04-28 09:00:00.000

(5 行受影响)
*/

------解决方案--------------------
 好纠结的人儿。。。
  SQL能处理的问题 非要搬到代码里面去执行
  按照你这需求 还不如弄个存储过程