Hey, not sure where else to post this...
I'm trying to group together a sql table that looks something like this:

PHP Code:
a_userID    |    b_userID     |    val
   1            |       2              |    30
   2            |       1              |    22
   1            |       3              |    47 
... such that I can get the SUM(val) of rows where a_userID=1 and b_userID=2 and vice versa. This is at the top of a massive query that I don't want to run twice or group further, so my thought was to generate a partner key for 1+2 which would be the same key as for 2+1, but would be unique to just that combination of partners... I'm somehow tongue-tied trying to express this more clearly...

There must be some simple equation that can generate a unique number for any two number combinations ............right? Or is that a completely daft question? For some reason this has me stumped.