Oracle SQL — DENSE_RANK

Jak zrozumiałem, tego potrzebujesz:

select client_ID, status_id, from_date, to_date, 
       sum(start_of_group) over (order by client_ID, from_date) + 1 rank
  from (SELECT c.client_ID, c.status_id, c.from_date, c.to_date,
               case when lag(c.client_ID, 1, c.client_ID) over (order by c.client_ID, c.from_date) = c.client_ID 
                     and lag(c.status_id, 1, c.status_id) over (order by c.client_ID, c.from_date) = c.status_id
                    then 0 else 1 end start_of_group
          FROM client c)
 order by client_ID, from_date

SQLFiddle