select
  NewsID,title,content,p_date,
  visitURL='item-'+ltrim(row_number()over(partition by p_date order by getdate()))
from
  News

------解决方案--------------------
SQL code

create table News(NewsID int,title varchar(8),content varchar(15),p_date date,visitURL varchar(6))

insert into News
select 1,'title1','content1','2011-10-18','' union all
select 2,'title1','content1','2011-10-18','' union all
select 3,'title1','content1','2011-10-18','' union all
select 4,'title1','content1','2011-10-18','' union all
select 5,'title1','content1','2011-10-19','' union all
select 6,'title1','content1','2011-10-19','' union all
select 7,'title1','content1','2011-10-19','' union all
select 8,'title1','content1','2011-10-19','' union all
select 9,'title1','content1','2011-10-19','' 
 
update a set a.visitURL='item-'+cast(b.s as varchar)
from News a
inner join
(select NewsID,row_number() over(partition by p_date order by NewsID) s from News) b
on a.NewsID=b.NewsID

select * from News
NewsID      title    content         p_date     visitURL
----------- -------- --------------- ---------- --------
1           title1   content1        2011-10-18 item-1
2           title1   content1        2011-10-18 item-2
3           title1   content1        2011-10-18 item-3
4           title1   content1        2011-10-18 item-4
5           title1   content1        2011-10-19 item-1
6           title1   content1        2011-10-19 item-2
7           title1   content1        2011-10-19 item-3
8           title1   content1        2011-10-19 item-4
9           title1   content1        2011-10-19 item-5