Define specialization. Explain disjoint constraint with example.

Specialization is a top-down process in the Enhanced ER (EER) model: an entity set is divided into sub-classes based on some distinguishing feature. Each sub-class inherits all the attributes and relationships of the super-class and can have its own extra attributes.

Example: the super-class EMPLOYEE(EmpID, Name, Salary) is specialised into SECRETARY(TypingSpeed), ENGINEER(EngineeringType) and TECHNICIAN(Grade). Every engineer is still an employee with an EmpID, Name and Salary.

(The opposite bottom-up process, combining similar entity sets into a super-class, is generalization.)

Disjoint constraint

The disjointness constraint says whether an entity of the super-class can belong to more than one sub-class.

• Disjoint (d): an entity can belong to at most one sub-class. In the ER diagram the specialization circle is marked d. Example: EMPLOYEE specialised by job type into SECRETARY, ENGINEER and TECHNICIAN. An employee does exactly one job, so the sub-classes are disjoint.
• Overlapping (o): an entity can belong to several sub-classes at once. The circle is marked o. Example: PERSON specialised into STUDENT and EMPLOYEE. A person can be both a student and an employee.

Disjointness is often combined with the completeness constraint (total or partial participation), for example "disjoint and total" when every employee must be exactly one of the listed types.