its not about loans,i have 2 tables,1for stock in hand which has 3 kind of reading books with their quantity of 2000 each,2nd table for books requested by all employees,in both tables i have relationship of book number,if someone request 100 books the stockinhand goes to 1900 and then if someone request another 200 books then it deducts from original quantity of 2000.and also someone request the same book number,it gives me error of valvue duplication.need your expert opinions please