--> 测试数据：[t1]
if object_id('[t1]') is not null drop table [t1]
create table [t1]([id] int,[ord_id] int)
insert [t1]
select 1,10 union all
select 2,10 union all
select 3,20 union all
select 4,20 union all
select 5,20

select id,(select min(id) from t1 a where a.[ord_id]=b.[ord_id])
 as [newid] ,[ord_id] from t1 b
group by [ord_id],id
/*
id    newid    ord_id
1    1    10
2    1    10
3    3    20
4    3    20
5    3    20
*/