Creating A New Column Of Differences (x[2]-x[1])

I have a table that looks like this:

id, x
1 9.5
2 10
3 11
...
n x

and what I would like is to update this table with the differences of values in the x column (x[2]-x[1]) where the first difference would just be defined as 0.

result table would look like this:

id, x, dx
1 9.5 0 (where dx[1] defined to be 0)
2 10 0.5 (where dx[2] = 10-9.5 = 0.5)
3 11 1.0 (where dx[3]= 11-10 = 1.0)
...

Any suggestions on the best way to do this?