For example, suppose you attempt to create a department, but supply a DepartmentID value that is already in use:
To do this, enter the command:
INSERT INTO Departments ( DepartmentID, DepartmentName, DepartmentHeadID ) VALUES ( 200, 'Eastern Sales', 902 );
The INSERT is rejected because the primary key for the table would no longer be unique. Since the DepartmentID column is a primary key, duplicate values are not permitted.