Question on Adding a New Column in Table

[Yu Xing Tay]

Suppose i have a table like this:

Name Value

A 20
A 25
B 30
B 46
B 72

What is the query i need to input to generate another column with the rule:
(if the name of the current row = name of prev row, then return current value - prev value. if the name of current row is not equal to name of prev row, return 0)

So the result will be:

Name Value Results

A 20 0
A 25 5
B 30 0
B 46 16
B 72 26

Thx a lot!