Is it possible to generate a unique random alphanumeric code consisting of 5 characters using Informix or standard sql?

I'm using informix IDS server, and I have this table table1 :

Column name          Type                                    Nulls

ad_code              char(5)                                 yes
ad_value             smallint                                yes

I would like to generate a random, unique code for field ad_code (and then store it in the ad_code field) of table table1 using sql. Is it possible ?