foreach (subscription) : 
    update snum column to customer_serialnumbers.serialnumber where:
      - the customer_serialnumbers.ordernum = subscription.ordernum
      - customer_serialnumbers.serialnumber is not yet consumed by another row in subscriptions
endforeach;

update		subscriptions 
set		snum = ( -- get a serialnumber from the customerserialnumber list 
			select	top 1 serialnumber
			from	customer_serialnumbers cs
			where	cs.ordernum = subscriptions.ordernum -- snum order same as subs order
				-- don't join to a revoked snum
				and	cs.active = 1
				-- snum not already used
				and	serialnumber not in (
					select	distinct(snum)
					from	subscriptions
					where	snum is not null
				)
		)
where		subscriptions.snum is null -- don't set something we've already set