Need help in writing stored procedure

I am assigning the location to the customer. Customer number and name kept in page header, location and checkbox kept in grid. I am storing the details with the following information: customer number, name, location and checkbox field. The constraint is, in grid under location field there are n number of location but user can assign only one location to the particular customer. After selecting first checkbox if user try to select another checkbox user should message that "You cant select multiple locationcode for the this customer", I mean user can not select more than one checkbox. It should happen the same while you are displaying the record. Suppose for cust1 you already assigned location(with checkboxmarked), while you are displaying cust1 information in the grid corresponding checkbox also be displayed. In this regard I would like to request to all expert to write one stored procedure to handle this situation…I want to handle this in back end. Pls help me somebody…..

Thanks,

Rahul