RESET a RANK

SQL> select * from t1;

      DEPT DATE1
---------- ---------
       100 01-JAN-13
       100 02-JAN-13
       200 03-JAN-13
       100 04-JAN-13

select dept,  
       date1,
       rank () Over (partition by dept order by date1) rnk
from t1
order by date1;

      DEPT DATE1            RNK
---------- --------- ----------
       100 01-JAN-13          1
       100 02-JAN-13          2
       200 03-JAN-13          1
       100 04-JAN-13          3

The desired output is as follows. The last rnk=1 is becuase the Jan-04 record is the first record after the change.

      DEPT DATE1            RNK
---------- --------- ----------
       100 01-JAN-13          1
       100 02-JAN-13          2
       200 03-JAN-13          1
       100 04-JAN-13          1  <<<----------

Solution:

select dept, date1,
       CASE WHEN StartFlag = 0 THEN 1
            ELSE 1+StartFlag+NVL(lag(StartFlag) over (order by date1),0)
       END as rnk
from (select t1.*,
             (case when dept = lag(dept) over (order by date1)
                   then 1
                   else 0
              end) as StartFlag
      from t1
     ) t1
order by date1;