Enhanced ER Model

Enhanced ER Model

Specialization and Generalization     AU: Dec- 19, Marks 7

• Some entities have relationships that form hierarchies. For instance, Employee can be an hourly employee or contracted employee.

• In this relationship hierarchies, some entities can act as superclass and some other entities can act as subclass.

• Superclass: An entity type that represents a general concept at a high level, is called superclass.

• Subclass: An entity type that represents a specific concept at lower levels, is called subclass.

• The subclass is said to inherit from superclass. When a subclass inherits from one or more superclasses, it inherits all their attributes. In addition to the inherited attributes, a subclass can also define its own specific attributes.

• The process of making subclasses from a general concept is called specialization. This is top-down process. In this process, the sub-groups are identified within an entity set which have attributes that are not shared by all entities.

• The process of making superclass from subclasses is called generalization. This is a bottom up process. In this process multiple sets are synthesized into high level entities.

• The symbol used for specialization/ Generalization is,                

• For example - There can be two subclass entities namely Hourly_Emps and Contract_Emps which are subclasses of Empoyee class. We might have attributes hours_worked and hourly wage defined for Hourly_Emps and an attribute contractid defined for ContractEmps.

Therefore, the attributes defined for an Hourly_Emps entity are the attributes for Employees plus Hourly_Emps. We say that the attributes for the entity set Employees are inherited by the entity set Hourly_Emps and that Hourly-Emps ISA (read is a) Employees. It can be represented by following Fig. 2.4.1.

Constraints on Specialization/Generalization

There are four types of constraints on specialization/generalization relationship. These are -

1) Membership constraints: This is a kind of constraints that involves determining which entities can be members of a given lower-level entity. There are two types of TO S membership constraints –

    i) Condition defined: In condition-defined lower-level entity sets,membership is evaluated on the basis of whether or not an entity satisfies an explicit condition or predicate. For example - Consider the high-level entity Set Employee that has attribute Employee_type. All Employee entities are evaluated on defining Employee_type attribute. All entities that satisfy the condition student type = "ContractEmployee" are included in Contracted Employee. Since all the lower-level entities are evaluated on the basis of the same attribute this type of generalization is said to be attribute-defined.

    ii) User defined: This is kind of entity set that in which the membership is manually defined.

2) Disjoint constraints: The disjoint constraint only applies when a superclass has more than one subclass. If the subclasses are disjoint, then an entity occurrence can be a member of only one of the subclasses. For entity Student has either Postgraduate Student entity or Undergraduate Student

3) Overlapping: When some entity can be a member of more than one subclasses. For example - Person can be both a Student or a Staff. The And can be used to représent this constraint.

4) Completeness: It specifies whether or not an entity in the higher-level entity set must belong to at least one of the lower-level entity sets within the generalization/specialization. This constraint may be one of the following –

   i) Total generalization or specialization: Each higher-level entity must belong to a lower-level entity set. For example - Account in the bank must either Savings account or Current Account. The mandatory can be used to represent this constraint.

    ii) Partial generalization or specialization: Some higher-level entities may not belong to any lower-level entity set.

Aggregation

A feature of the entity relationship model that allows a relationship set to participate in another relationship set. This is indicated on an ER diagram by drawing a dashed box around the aggregation.

For example - We treat the relationship set work and the entity sets employee and project as a higher-level entity set called work.

Review Question

1. Explain with suitable example, the constraints of specialization and generalization in ER modeling.  AU: Dec. 19, Marks 7