How can I calculate differences between values

Hello there!

I have a table foo( userid integer, data integer); with the
constraint unique(userid,data)

Now I wish to select one userid and calculate the differences between
the data -values (when they are sorted) .
For example
if the table contains:

4, 100
5, 200
5, 210
5, 231

I want the values

5,10
5,21

what should happen to the 4,100 record you may ask, I will try to
exclude that case by requireing each userid to have at least two data
values.

The question is now: is there a clever and efficient way of
calculating these differences in data values?

The only thing I can think of is picking a data value and
select min(data) from foo where userid=5 and data>200
and then calculate the difference and do what I want with it, and the
repeat this process with the last selected data value.
I guess that will work, but I'm curious, are there other ways?