i mean 

10
5
3
1 

are already in the B Column?

Table Structre
--------------
Name  Null?  Datatype
-------------
A     Not Null  number
B               number


Table Data
---------------
A   |   B  |
----|------|-----
5   |      |
2   |      |
2   |      |
1   |      |
-----------------------------
Expected Output
---------------------------
A   |   B  |
----|-------
5   |   5
2   |   3
2   |   1
1   |   0 
----|------|-----------------

select
  a
,b-a b
from sampl1
order by a desc

SQL> desc m_test;
 Name                                      Null?    Type
 ----------------------------------------- -------- --------------
 A                                                  NUMBER
 B                                                  NUMBER

SQL> select * from m_test;

         A          B
---------- ----------
         5
         2
         2
         1

SQL> 

SCOTT@orcl12c> SELECT * FROM m_test
  2  /

         A          B
---------- ----------
         5
         2
         2
         1

4 rows selected.

SCOTT@orcl12c> SELECT a,
  2  	    10 - SUM (a) OVER (ORDER BY a DESC, ROWID) b
  3  FROM   m_test
  4  /

         A          B
---------- ----------
         5          5
         2          3
         2          1
         1          0

4 rows selected.

SQL> desc m_test;
ERROR:
ORA-04043: object m_test does not exist


SQL> select * from m_test;
select * from m_test
              *
ERROR at line 1:
ORA-00942: table or view does not exist

sql> Create table m_test
     (a number, b number);

SUM_A is the summary column
declare
      v_sum  number := 0 ;    
begin
      go_block('block');
      first_record ;
      loop
            v_sum:=  v_sum+ :a ;
            :b   :=  :SUM_A - v_sum;
            exit when :system.last_record = 'TRUE' ;
            next record ;
      end loop ;
end ;

SQL> Create table m_test
  2       (a number, b number);

Table created.

SQL> select * from m_test;

no rows selected

sql> insert into m_test (a) values(5);

sql> insert into m_test (a) values(2);

sql> insert into m_test (a) values(2);

sql> insert into m_test (a) values(1);

SQL> insert into m_test (a) values(1);

1 row created.

SQL> insert into m_test (a) values(2);

1 row created.

SQL> insert into m_test (a) values(2);

1 row created.

SQL> insert into m_test (a) values(5);

1 row created.

SQL> commit;

Commit complete.

SQL> select * from m_test;
         A          B
---------- ----------
         1
         2
         2
         5

4 rows selected.

SQL> with total as (select sum(a) total from m_test)
  2  select a,
  3         total - sum(a) over (order by a desc) b
  4  from m_test, total
  5  order by a desc
  6  /
         A          B
---------- ----------
         5          5
         2          1
         2          1
         1          0

4 rows selected.

SQL> create or replace view v as
  2  with total as (select sum(a) total from m_test)
  3  select a,
  4         total - sum(a) over (order by a desc) b
  5  from m_test, total
  6  /

View created.

SQL> select * from v order by a desc;
         A          B
---------- ----------
         5          5
         2          1
         2          1
         1          0

4 rows selected.