The only way you're going to ENSURE that it's unique is to keep track of all of the ones previously generated. Use whatever methodology you like to gen it, then store it in table: if it's already been gen'd then do it again.
----------------------------------------------------------------------------------
Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?