The Extended Entity Relationship (EER) model is an expanded version of the standard ER model. As databases became more complex, the basic ER model was insufficient to represent real world systems like CAD software, geographical systems, and medical databases.
To handle these requirements, the EER model keeps all basic ER concepts (entities, attributes, relationships) and adds advanced semantic modeling constructs:
Key Features of the EER Model
The Enhanced Entity-Relationship (EER) Model is an extended version of the basic ER model. It provides additional concepts to represent complex relationships and real-world situations more clearly.
1. Subclasses and Superclasses
A superclass is a general entity type that contains common attributes and relationships, while a subclass is a subgroup of the superclass with its own specific characteristics.
For example, EMPLOYEE can be a superclass, while SECRETARY, ENGINEER, and MANAGER can be its subclasses.
This allows us to represent different types of employees while keeping their common information in the EMPLOYEE superclass.
2. Specialization and Generalization
Specialization is a top-down approach that breaks down a superclass into more specific subclasses based on their distinctive characteristics.
Generalization is a bottom-up approach that combines similar subclasses into a common superclass based on their shared characteristics.
Both concepts help organize entities and reduce unnecessary repetition of common information.
3. Attribute Inheritance
Attribute inheritance means that a subclass automatically receives or inherits the attributes and relationships of its superclass.
For example, if EMPLOYEE has attributes such as Emp_ID and Name, the ENGINEER subclass automatically inherits these attributes.
The subclass can also have its own additional attributes that are specific to it.
4. Category (Union Type)
A Category or Union Type represents a subclass whose members can come from two or more different superclasses.
For example, an OWNER entity can be a PERSON, BANK, or COMPANY, meaning that an owner can belong to any one of these entity types.
This feature is useful when different types of entities need to participate in the same role or relationship.
5. Aggregation
Aggregation allows a relationship set to be treated as an entity set so that it can participate in another higher-level relationship.
It is useful for representing complex situations where a relationship itself needs to be connected to another entity or relationship.
For example, the Works_On relationship between EMPLOYEE and PROJECT can be treated as a higher-level unit and related to another entity such as MANAGER.