on change of column update one column

Hi friends,

I need your help to update the table.

I have table called on_change

f1 number(5),
f2 number(5),
f3 number(5),
f4 number(5)

my data is look like as below:

F1 F2 F4
---------- ---------- ----------
1 0 0
1 0 3
1 0 7
1 1 1
1 1 2
1 1 3
1 2 0
1 2 1
1 2 2
1 2 3
1 2 4
1 7 0
1 7 1
1 7 2
1 7 3
1 15 1
1 15 2
1 15 3
1 15 4
2 0 0
2 0 3
2 0 7
2 1 1
2 1 2
2 1 3
2 1



I want to update on_change table in following way:

- first I want order in f1, f2, f4 with group f1+f2
- when f1 +f2 changes I want to update f4 with 1,2,3,4

Expected result of above data should be as below:


F1 F2 F4
---------- ---------- ----------
1 0 1
1 0 2
1 0 3

1 1 1
1 1 2
1 1 3

1 2 1
1 2 2
1 2 3
1 2 4
1 2 5

1 7 1
1 7 2
1 7 3
1 7 4

1 15 1
1 15 2
1 15 3
1 15 4

2 0 1
2 0 2
2 0 3

2 1 1
2 1 2
2 1 3


The idea is to update F4 when there is change in f1 and f2 with incremental value 1,2,3....

Can anybody suggest me possible pl-sql block for this?

Thanks in advance.