Enhanced ER Model

As data complexity grows, the traditional ER model becomes less effective for database modeling. Enhanced ER diagrams extend the basic ER model to better represent complex applications. They support advanced concepts like subclasses, generalization, specialization, aggregation, and categories.

ER model

The ER model is the abstract representation of a database structure that defines

• Entities in a database.
• Attributes that they had.
• Relationships between them.

What is an Enhanced ER model?

Enhanced ERMs are high-level models that represent the requirements and complexities of complex databases. The EER model includes all modeling concepts of the ER model. In addition, EER includes the following concepts.

• Subclasses and Superclasses
• Specialization and Generalization
• Category or Union type
• Attribute and Relationship Inheritance

Superclass and Subclass

A superclass is a high-level entity that can be further segmented into subclasses or subsets. It is also referred to as a Parent class. A subclass can be referred to as a child or derived class.

Example: Science is a Super class which has subclasses like Physics, Chemistry, Biology.

Generalization and Specialization

Generalization and Specialization are common relationships added as enhancements to the classical ER model. A subclass (specialized class) inherits from a superclass (generalized class), similar to object-oriented concepts. This is best understood using IS-A relationships like “Technician IS-A Employee” or “Laptop IS-A Computer.”

An entity is a specialized type/class of another entity. For example, a Technician is a special Employee in a university system Faculty is a special class of Employees. We call this phenomenon generalization/specialization. Here Employee is a generalized entity class while the Technician and Faculty are specialized classes of Employee. 

Example:

This example instance of "sub-class" relationships. Here we have four sets of employees: Secretary, Technician, and Engineer. The employee is a super-class of the rest three sets of individual sub-class is a subset of Employee set.

• An entity belonging to a sub-class is related to some super-class entity. For instance emp, no 1001 is a secretary, and his typing speed is 68. Emp no 1009 is an engineer (sub-class) and her trade is “Electrical”, so forth.
• Sub-class entity “inherits” all attributes of super-class; for example, employee 1001 will have attributes eno, name, salary, and typing speed.

Enhanced ER Model of Above Example 

Constraints

There are two types of constraints on the “Sub-class” relationship.  

Total or Partial Sub-classing

• Total: Every entity in the superclass must be in at least one subclass (e.g., every employee is either salaried or hourly).
• Partial: Some entities may not belong to any subclass (e.g., not all employees are a secretary, engineer, or technician).
• Total subclassing means complete coverage; partial means incomplete coverage.

Overlapped or Disjoint Sub-Classing

• Overlapped: An entity can belong to multiple subclasses.
• Disjoint: An entity can belong to only one subclass.
• In the given examples, both job-type and salary-based subclassing are disjoint, meaning no overlap.

Note - These constraints are independent of each other: can be "overlapped and total or partial" or "disjoint and total or partial". Also, sub-classing has transitive properties. 

Multiple Inheritance (Sub-Class of Multiple Superclasses)

An entity can be a sub-class of multiple entity types such entities are sub-class of multiple entities and have multiple super-classes. In multiple inheritances, attributes of sub-class are the union of attributes of all super-classes. 

Example: Teaching Assistant can subclass of Employee and Student both. A faculty in a university system can be a subclass of Employee and Alumnus.

Union

• Set of Library Members is UNION of Faculty, Student, and Staff. A union relationship indicates either type; for example, a library member is either Faculty or Staff or Student.
• Below are two examples that show how UNION can be depicted in ERD – Vehicle Owner is UNION of PERSON and Company, and RTO Registered Vehicle is UNION of Car and Truck.

There might be some confusion in Sub-class and UNION.

Consider an example in above figure Vehicle is super-class of CAR and Truck. In the example, Vehicle is a superclass of Car and Truck, which normally implies inheritance of attributes. However, in the RTO-registered case, Car and Truck form a union without inheriting from Vehicle, each has independent attributes.

An Enhanced Entity-Relationship (EER) model is an extension of the original Entity-Relationship (ER) model that includes additional concepts and features to support more complex data model requirements. It supports complex features, such as subtypes and supertypes, generalization and specialization, and inheritance.

Key Features of the EER Model

• Subtypes and Supertypes: The EER model allows for the creation of subtypes and supertypes. It allows the creation of a hierarchy where a supertype represents general attributes and subtypes represent specialized entities (e.g., Vehicle → Car, Truck).
• Generalization and Specialization: Generalization is the process of identifying common attributes and combines common features into a supertype, while Specialization is the process of defining subtypes with unique attributes from a supertype.
• Inheritance: Inheritance is a mechanism that allows subtypes to inherit attributes and relationships from their supertype. This means that any attribute or relationship defined for a supertype is automatically inherited by all its subtypes.
• Constraints: The EER model allows for the specification of constraints that must be satisfied by entities and relationships. Examples of constraints include cardinality constraints, which specify the number of relationships that can exist which tells whether an entity is required to participate in a relationship.
• Subclasses and Superclasses: EER model allows for the creation of a hierarchical structure of entities where a superclass can have one or more subclasses. Each subclass inherits attributes and relationships from its superclass, and it can also have its unique attributes and relationships.
• Attribute Inheritance: EER model allows attributes to be inherited from a superclass to its subclasses. This means that attributes defined in the superclass are automatically inherited by all its subclasses.
• Union Types: E ER model allows for the creation of a union type, which is a combination of two or more entity types. The union type can have attributes and relationships that are common to all the entity types that make up the union.
• Aggregation: EER model allows for the creation of an aggregate entity that represents a group of entities as a single entity. The aggregate entity has its unique attributes and relationships.
• Multi-valued Attributes: EER model allows an attribute to have multiple values for a single entity instance. For example, an entity representing a person may have multiple phone numbers.
• Relationships with Attributes: EER model allows relationships between entities to have attributes. These attributes can describe the nature of the relationship or provide additional information about the relationship.

Overall, these features make the EER model more expressive and powerful than the traditional ER model, allowing a more accurate representation of complex relationships between entities.