with t1 as
(
     select '2012/10/17 8:00:00' c1,'db1' name,20 used from dual
     union all
     select '2012/10/17 8:30:00' c1,'db1' name,23 used from dual
     union all
     select '2012/10/17 9:00:00' c1,'db1' name,32 used from dual
     union all
     select '2012/10/18 8:00:00' c1,'db1' name,15 used from dual
     union all
     select '2012/10/18 8:30:00' c1,'db1' name,18 used from dual
     union all
     select '2012/10/18 9:00:00' c1,'db1' name,21 used from dual
     union all
     select '2012/10/18 10:00:00' c1,'db1' name,14 used from dual
     union all
     select '2012/10/19 8:00:00' c1,'db1' name,23 used from dual
     union all
     select '2012/10/19 10:00:00' c1,'db1' name,17 used from dual
)

select c1,name,used,
       round(used-lag(used) over(partition by name order by rownum),2) z1,
       round(used-lag(used) over(partition by to_char(to_date(c1,'yyyy/mm/dd hh24:mi:ss'),'hh24:mi:ss') order by c1),2) z2
from t1
order by to_date(c1,'yyyy/mm/dd hh24:mi:ss')


            c1         name  used   z1   z2
---------------------------
1    2012/10/17 8:00:00    db1    20        
2    2012/10/17 8:30:00    db1    23    3    
3    2012/10/17 9:00:00    db1    32    9    
4    2012/10/18 8:00:00    db1    15    -17    -5
5    2012/10/18 8:30:00    db1    18    3    -5
6    2012/10/18 9:00:00    db1    21    3    -11
7    2012/10/18 10:00:00    db1    14    -7    
8    2012/10/19 8:00:00    db1    23    9    8
9    2012/10/19 10:00:00    db1    17    -6    3