Doubt in writing a function

Question

i have a table like the following
C1     C2      C3
1     1/21/2005     200
1     1/25/2005     300
1     2/22/2005     500
2     4/20/2005     500
2     3/24/2005     5000
I have to write a function where i pass a number.
The function should search C1 for that number and compare the last 2 entries based on date for same Number in C1
this should return teh difference of the last 2 entries for a particular value in C1
For eg
C1     C2          C3
1     1/22/2005     200
1     2/24/2005     300
Since there is a increase comparing the last 2 entries of C1 the output should be
the last value in C2 and the difference of 100
Pls help me wiht a similar function for this
If

Answer 1

Like This..?
SQL> select *
  2  from test;
        C1 C2                C3                                                
         1 21-JAN-05        200                                                
         1 25-JAN-05        300                                                
         1 28-JAN-05        500                                                
         2 28-JAN-05        500                                                
SQL> select c1,c2,c3,diff
  2  from(
  3  select c1,c2,c3,row_number() over(partition by c1 order by c2 desc) rn,
  4           c3-lead(c3) over(partition by c1 order by c2 desc) diff
  5  from test)
  6  where rn=1
  7  and c1=1;
        C1 C2                C3       DIFF                                     
         1 28-JAN-05        500        200