You are the DB Designer for ABC International Airport's Airport Management Center. 

You and your colleagues design and maintain the database backbone used by the various departments in the airport. 
You and your fellow colleague, Steve, are responsible for designing and maintaining all databases that pertain to reservations and flights. 
You want to include a PassportNumber column in the Passengers table so that the passenger's passport number can be used when placing future reservations. 
No two passengers have the same passport number the world round. Thus, you want information in that column to be strictly unique. 
Information in the Passengers table has already been made unique by the PassengerID PRIMARY KEY column. 

What constraint would you use to enforce the uniqueness of the PassportNumber column?

You can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that do not participate in a primary key. Although both a UNIQUE constraint and a PRIMARY KEY constraint enforce uniqueness, use a UNIQUE constraint instead of a PRIMARY KEY constraint when you want to enforce the uniqueness of a column, or combination of columns, that is not the primary key.
Multiple UNIQUE constraints can be defined on a table, whereas only one PRIMARY KEY constraint can be defined on a table.
Also, unlike PRIMARY KEY constraints, UNIQUE constraints allow for the value NULL. However, as with any value participating in a UNIQUE constraint, only one null value is allowed per column.