Database Software

SQL> with T as (
  2  select  to_date('2011-03-26','YYYY-MM-DD') as DT, 'a' as usr from dual union
  3  select to_date('2011-03-26','YYYY-MM-DD') , 'b'  from dual union
  4  select to_date('2011-03-27','YYYY-MM-DD') , 'b' from dual union
  5  select to_date('2011-03-27','YYYY-MM-DD') , 'c' from dual union
  6  select to_date('2011-03-28','YYYY-MM-DD') , 'c' from dual union
  7  select to_date('2011-03-28','YYYY-MM-DD') , 'd' from dual union
  8  select to_date('2011-03-29','YYYY-MM-DD') , 'd' from dual union
  9  select to_date('2011-03-29','YYYY-MM-DD') , 'e' from dual )
 10  SELECT a.dt, COUNT(DISTINCT b.usr)
 11  FROM
 12    (SELECT DISTINCT DT FROM T ) A,
 13    T B
 14  WHERE A.DT>=b.dt
 15  GROUP BY a.dt
 16  order by a.dt;

DT       COUNT(DISTINCTB.USR)
-------- --------------------
26-03-11                    2
27-03-11                    3
28-03-11                    4
29-03-11                    5

with t(DT,USR) as(
select date '2011-03-26','a' from dual union all
select date '2011-03-26','b' from dual union all
select date '2011-03-27','b' from dual union all
select date '2011-03-27','c' from dual union all
select date '2011-03-28','c' from dual union all
select date '2011-03-28','d' from dual union all
select date '2011-03-29','d' from dual union all
select date '2011-03-29','e' from dual)
select DT,sum(sum(willSum)) over(order by DT) as "distinct user"
from (select DT,USR,
      case Row_Number() over(partition by USR order by DT)
      when 1 then 1 else 0 end as willSum
      from t)
group by DT
order by DT;

DT        distinct user
--------  -------------
11-03-26              2
11-03-27              3
11-03-28              4
11-03-29              5

SELECT dt,
       (SELECT COUNT(DISTINCT(usr)) 
          FROM t t1 
         WHERE t1.dt <= t.dt) as distinct_users 
FROM t
GROUP BY dt
ORDER BY dt

DT        DISTINCT_USERS
--------- --------------
26-MAR-11              2
27-MAR-11              3
28-MAR-11              4
29-MAR-11              5

WITH	got_r_num	AS
(
	SELECT	dt
	,	ROW_NUMBER () OVER ( PARTITION BY usr
				     ORDER BY	  dt
				   )	AS r_num
	FROM	t
--	WHERE	...	-- If you need any filtering, put it here
)
SELECT DISTINCT
	  dt
,	  COUNT (*) OVER (ORDER BY dt)	AS distinct_user
FROM	  got_r_num
WHERE	  r_num	= 1
ORDER BY  dt
;