how to get remaing quantity in shoping cart after shope

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C#

ASP.NET

LINQ

C#4.0

i have two tables in database like

checkout_id  /   product_id   /   product_name  /   quantity  /  unit_price   /   user_name
    1                 54           gh                 3             123               oman
    2                 55           fgh                5             900            choudhary
    3                 56           try                6             809              x-men  

above is my check out table which have the complete information of last checkout
below is my book_detail table

book_id    /     quantity     /   book_name   /   price
   53              20                asley         800
   54              10               janmes         123
   55              16               horror         900
   56              25                divine        809
   57              30                trumph         555
   58              40                 dekh          788

now i want to find remaing quantity and wants to update in book_detail table

so how to solve this problem i am not getting proper solution bcz i m new on .net platform
as well as sql
i have tried this but its not enough or not good

var checkoutbook = from st in obj.Checkout_tables
                             where st.User_name== Session["user"].ToString()
                             select st.Product_id;

             var bookqan  = from st in obj.Book_details
                            where checkoutbook.Contains(st.Book_Id)
                            select st.Quantity;
             var bookid = from st in obj.Book_details
                           where checkoutbook.Contains(st.Book_Id)
                           select st.Book_Id;
             Int32 z = checkoutbook.Count();
             for (int s = 0; s < z; s++)
             {
                 Int32 st1 = (from st in obj.Book_details
                           where st.Book_Id == checkoutbook[s]
                           select st.Quantity).SingleOrDefault();

             }