Mysql cumulated sum

Sometimes we need to obtain a cumulative sum of a columns in a query. How we can do it?
Suppose you have this simple table

id|in|out
----------
1  5  0
2  0  6
3  2  0
4  3  0
5  0  4

and you want obtain the cumulative sum  of the column (in+out) like this

id|in|out|sum
-------------
1  5  0   5
2  0  6   11
3  2  0   13
4  3  0   16
5  0  4   20

This following query to do this

SELECT a.id, a.in, a.out,

-- this is the new column called sum
( SELECT SUM( in + out )
  FROM mytable b
  WHERE b.id <= a.id

) AS sum

FROM mytable a
ORDER BY id