Oracle SQL - DENSE_RANK

Question

I have a Client data table, selected columns of which are shown below:

Row_ID  Client_ID  Status_ID  From_date           To_date
  1     123456       4               


        

Answer 1

Simple mark all status changes for each client with 1 (column GRP below). Than add those number using analytic SUM function:

with tab1 as (
select client_id,from_date, status,
 nvl(lag(status) over (partition by client_id  order by from_date),-1) status_lag,
 case when (nvl(lag(status) over (partition by client_id  order by from_date),-1) <> status) then 
 1 end grp
from tst
)
, tab2 as (
select client_id,from_date, status,status_lag, grp,
sum(grp) over (partition by client_id  order by from_date) as RANK
from tab1
)
select * from tab2;

gives as expected

 CLIENT_ID FROM_DATE               STATUS STATUS_LAG        GRP       RANK
---------- ------------------- ---------- ---------- ---------- ----------
      1001 01.10.2015 00:00:00          1         -1          1          1 
      1001 02.10.2015 00:00:00          1          1                     1 
      1001 03.10.2015 00:00:00          2          1          1          2 
      1001 04.10.2015 00:00:00          2          2                     2 
      1001 05.10.2015 00:00:00          3          2          1          3 
      1001 09.10.2015 00:00:00          1          3          1          4 
      1002 12.10.2015 00:00:00          1         -1          1          1 
      1002 13.10.2015 00:00:00          3          1          1          2 
      1002 15.10.2015 00:00:00          3          3                     2

my setup

 create table tst 
 (client_id number,
 from_date date,
 status number);

 insert into tst values (1001, to_date('01-10-15','dd-mm-rr'),1);
 insert into tst values (1001, to_date('02-10-15','dd-mm-rr'),1);
 insert into tst values (1001, to_date('03-10-15','dd-mm-rr'),2);
 insert into tst values (1001, to_date('04-10-15','dd-mm-rr'),2);
 insert into tst values (1001, to_date('05-10-15','dd-mm-rr'),3);
 insert into tst values (1001, to_date('09-10-15','dd-mm-rr'),1);
 insert into tst values (1002, to_date('12-10-15','dd-mm-rr'),1);
 insert into tst values (1002, to_date('13-10-15','dd-mm-rr'),3); 
 insert into tst values (1002, to_date('15-10-15','dd-mm-rr'),3);
 commit;