with
       t_table as
     (
       select to_date('2006-01-01', 'yyyy-mm-dd') as a, 'WWF011' as b from dual union all
       select to_date('2006-02-03', 'yyyy-mm-dd') as a, 'WWF011' as b from dual union all
       select to_date('2006-05-04', 'yyyy-mm-dd') as a, 'WWF011' as b from dual union all
       select to_date('2006-02-02', 'yyyy-mm-dd') as a, 'WWF022' as b from dual union all
       select to_date('2006-05-04', 'yyyy-mm-dd') as a, 'WWF022' as b from dual
     )
select a
     , b
  , a - nvl(lag (a) over (partition by b order by a), a) as days
  from t_table