SQL and ippool

Hello all, The scenario will be: when Freeradius receives a user request, it forwards it to SQL server (local or remote) for authenticating the username/password. If the username/password is valid, then Freeradius will use a certain ippool to assign IP address to the user, based on username. Can this be accomplished? I am wondering, when SQL sends a reply back to Freeradius, how can Freeradius knows what ippool it should use to assign IP based on user name? Can anyone shed some light on it? Thanks in advance