You administer a Microsoft SQL Server 2012 database. The database contains a table named Employee.
Part of the Employee table is shown in the exhibit.


Confidential information about the employees is stored in a separate table named EmployeeData. One record exists within EmployeeData for each record in the Employee table. You need to assign the appropriate constraints and table properties to ensure data integrity and visibility. On which column in the Employee table should you create a unique constraint?