One-to-one relationship
A one-to-one relationship exists when each row in one table has only one related row in a second table. For example, a business might decide to assign one office to exactly one employee. Thus, one employee can have only one office. The same business might also decide that a department can have only one manager. Thus, one manager can manage only one department.
Figure 2–2 shows this relationship.
The business might also decide that for one office there can be zero or one employee, or for one department there can be no manager or one manager. These relationships are described as zero-or-one relationships.