create table tb_text
(datetime1 varchar(50))

insert into tb_text (datetime1)
select '2012-08-12,13:40:30' union all
select '2012-08-12,13:41:30' union all
select '2012-08-12,13:42:30' union all
select '2012-08-12,13:43:30' union all
select '2012-08-12,13:44:30' union all
select '2012-08-12,13:45:30' union all
select '2012-08-12,14:01:30' union all
select '2012-08-12,14:02:30' union all
select '2012-08-12,14:03:30' union all
select '2012-08-12,14:04:30' union all
select '2012-08-12,14:05:30'
/*
（所影响的行数为 11 行）
*/
select * from tb_text
/*
2012-08-12,13:40:30
2012-08-12,13:41:30
2012-08-12,13:42:30
2012-08-12,13:43:30
2012-08-12,13:44:30
2012-08-12,13:45:30
2012-08-12,14:01:30
2012-08-12,14:02:30
2012-08-12,14:03:30
2012-08-12,14:04:30
2012-08-12,14:05:30

*/
update tb_text set datetime1 = replace(datetime1,',',' ')
/*（所影响的行数为 11 行）*/
select * from tb_text
/*

2012-08-12 13:40:30
2012-08-12 13:41:30
2012-08-12 13:42:30
2012-08-12 13:43:30
2012-08-12 13:44:30
2012-08-12 13:45:30
2012-08-12 14:01:30
2012-08-12 14:02:30
2012-08-12 14:03:30
2012-08-12 14:04:30
2012-08-12 14:05:30

*/
alter table tb_text
 alter column datetime1 datetime

select * from tb_text
/*
2012-08-12 13:40:30.000
2012-08-12 13:41:30.000
2012-08-12 13:42:30.000
2012-08-12 13:43:30.000
2012-08-12 13:44:30.000
2012-08-12 13:45:30.000
2012-08-12 14:01:30.000
2012-08-12 14:02:30.000
2012-08-12 14:03:30.000
2012-08-12 14:04:30.000
2012-08-12 14:05:30.000
*/

select max(datetime1) from tb_text group by convert(char(13),datetime1,121)

/*
2012-08-12 13:45:30.000
2012-08-12 14:05:30.000
*/