không phải bởi bất kỳ điều kiện có thể được đánh giá tự động. Hai hạn chế khác có thể áp dụng đối với một chuyên môn. Đầu tiên là hạn chế disjointness, trong đó nêu rõ rằng các lớp con chuyên môn phải được phân chia. Điều này có nghĩa là một thực thể có thể là một thành viên của tối đa là một trong các lớp con của chuyên môn. | Constraints and Characteristics of Specialization and Generalization 93 When we do not have a condition for determining membership in a subclass the subclass is called user-defined. Membership in such a subclass is determined by the database users when they apply the operation to add an entity to the subclass hence membership is specified individually for each entity by the user not by any condition that may be evaluated automatically. Two other constraints may apply to a specialization. The first is the disjointness constraint which specifies that the subclasses of the specialization must be disjoint. This means that an entity can be a member of at most one of the subclasses of the specialization. A specialization that is attribute-defined implies the disjointness constraint if the attribute used to define the membership predicate is single-valued. Figure 4-4 illustrates this case where the d in the circle stands for disjoint. We also use the d notation to specify the constraint that user-defined subclasses of a specialization must be disjoint as illustrated by the specialization hourly_employee salaried_employee in Figure . If the subclasses are not constrained to be disjoint their sets of entities may overlap that is the same real-world entity may be a member of more than one subclass of the specialization. This case which is the default is displayed by placing an o in the circle as shown in Figure . The second constraint on specialization is called the completeness constraint which may be total or partial. A total specialization constraint specifies that every entity in the superclass must be a member of at least one subclass in the specialization. For example if every EMPLOYEE must be either an HOURLY_EMPLOYEE or a SALARIED_EMPLOYEE then the specialization hourly_employee salaried_employee of Figure is a total specialization of EMPLOYEE. This is shown in EER diagrams by using a double line to connect the superclass to the circle. A single line is
