When to Use Joined Columns?

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Joined columns are especially useful when foreign keys are not meaningful to the end-user. In the Travel Agency system, for example, this is the case where numbers are used to identify the person who made the reservation. These numbers do not mean much to an end-user, but he or she would know which person is meant if the ID is accompanied by the family name. To achieve this, you include the family name column from the Person table as a joined column in the Reservation table.