I have a database with columns looking like:

session | order | atype | amt
--------+-------+-------+-----
1       |  0    | ADD   | 10
1       |  1    | ADD   | 20
1       |  2    | SET   | 35
1       |  3    | ADD   | 10
2       |  0    | SET   | 30
2       |  1    | ADD   | 20
2       |  2    | SET   | 55

It represents actions happening. Each session starts at 0. ADD adds an amount, while SET sets it. I want a function to return the end value of a session, e.g.

SELECT session_val(1); --returns 45
SELECT session_val(2); --returns 55

Is it possible to write such a function/query? I don't know how to do any iteration-like things with SQL, or if it's possible at all.