;with f as
(
select
  convert(varchar(10),b.loginTime,120) as 日期,isnull(count(distinct a.username),0) as 人数,a.username as 登入人
from
  user a left join task b
on
  a.taskID=b.taskID
)
select
 日期,人数, [登入人]=stuff((select ','+[value] from f where 日期=t.日期 for xml path('')), 1, 1, '') 
from
 f t 
group by
 日期,人数

------解决方案--------------------
SQL code
create table [user](username varchar(10),taskid int)
insert into [user]
select '张三',1 union
select '张三',2 union
select '张三',3 union
select '李四',4
create table task(taskid int,loginTime datetime)
insert into task
select 1,'2012-12-23 11:30:00' union
select 2,'2012-12-23 18:30:00' union
select 3,'2012-12-25 11:30:00' union
select 4,'2012-12-23 11:00:00' 

declare @bdate datetime=(select min(loginTime) from task),
        @edate datetime=(select MAX(loginTime) from task)
;with cte1 as
(
  select 日期=DATEADD(day,number,@bdate) from master.dbo.spt_values
     where type='p' and number<=DATEDIFF(DAY,@bdate,@edate)
 )
 ,cte2 as
 (
   select a.username,b.loginTime from [user] a,task b where a.taskid=b.taskid
 )
 
select 日期,
       人数=(select COUNT(distinct username) from cte2 where DATEDIFF(DAY,a.日期,loginTime)=0),
       登入人=isnull(stuff((select distinct ','+username from cte2 
             where DATEDIFF(DAY,a.日期,loginTime)=0 for xml path('')),1,1,''),'无')
from cte1 a      

/*
日期                      人数          登入人
----------------------- ----------- -------------
2012-12-23 11:00:00.000 2           李四,张三
2012-12-24 11:00:00.000 0           无
2012-12-25 11:00:00.000 1           张三