ID   NUMBER
  1     20100101 
  2     20100201 
  3     20100301 
  5     20100101 
  6     20100201 

ID   NUMBER
  1     20100101 
  2     20100201 
  3     20100301 
  5     20100101 
  6     20100201 
  0     20100301 

declare @t table (idx int, Number datetime)

insert @t
 select  1 , '20100101' union all 
 select  2 , '20100201' union all
 select  3 , '20100301' union all
 select  5 , '20100101' union all
 select  6 , '20100201'

select * from @t

declare @tmp table (num int identity, Number datetime)
;with cte as
(
 select count(*) [kolvo], Number from @t group by Number
)
insert @tmp
 select cte.Number from cte
 cross join (select top (select max(kolvo) from cte)  1  as a from INFORMATION_SCHEMA.COLUMNS) b
--select * from @tmp tmp

;with cte as
(
 select *, row_number() over (order by (select  1 )) num from @t
)
select isnull(cte.idx, 0 ) [idx], isnull(cte.Number,tmp.Number) [Number] from @tmp tmp
left join cte on (tmp.num = cte.num)