1) Null Value In Foreign Key 2) Extra Value To Indicate All The Values In PK Table Apply

I have two tables  1) tblCustomer (ID, Name, City)  2) tblemp (ID, NAME, Dept.ID, tblcustomer.ID)

Both the tables have ID as PK
A emp can be either assigned a) All customers b)single customer c) NO customer

Pls note:- there will never be 2 or 3 customer linked to emp (my actuall requirement tables are different but to explain i am using the above tables)

I know how to assign single customer......but had problem how to link all customers and "no customer"

Please tell me if the following solution is right?
1) I will manually insert a record in tblCustomer with id 0 as " all customers" and will not allow the user to delete it
2) store null in FK if it is "no customer"


Also please tell me is it ok to store one more value in tblCustomer as -1 and take it as "No customer"

I have a DDL in the Employee page which should be displaying the names of allthe customers with 2 extra values "ALL CUSTOMERS" and "SELECT"  Select is the default value in DDL which says "NO CUSTOMER" selected yet

Thank you
Sara