value   rate
100      0  (100-100)
90       10 (100-90) 
70       20 (90-70)
50       20 (70-50)
10       40 (50-10)

im having a table with two columns 'value' and 'rate'
in the value column im having the values . now i want to update the rate column as per i mentioned above
(value in 1st row - value in second row)

please tell me how to do it?