SQL Server can guarantee unique values in two ways, Primary keys and a unique indexes/constraints. The primary difference between the two is that fields bound to a unique index or constraint can be NULL while a primary key field can not. For the purpose of the unique indexes and constraints, NULL values are treated equal. That means that you cannot have two NULL values. Fortunately, there is a workaround for that will allow you to store multiple NULL values while ensuring that non-NULL values are...