I've done db Layer. While coding HotelBusiness in business Layer, I am not sure about such a thing. In my book, i call isBooked() between data.lock and data.update(). In my isBooked() i plan to check whether the customerid is null. Is it right? If it is, my problem is that in my completed code there is no such a step will remove a customerid after it once added. Please help me.