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one-to-one relationship

An association between two tables in which the primary key value of each record in the primary table corresponds to the value in the matching field or fields of one and only one record in the related table.

This type of relationship is unusual. An example is the relationship between an Employees table and a Mailboxes table used to store information about employees' e-mail addresses. The tables have a one-to-one relationship because each employee has only one mailbox, and each mailbox is assigned to only one employee.