Скачать или смотреть What is an Extended Entity-Relationship model? Superclasses | Subclasses | Union type | Aggregation

Welcome back MechanicaLEi, did you know that Enhanced Entity RElationship or EER model was developed to precisely reflect constraints in complex databases such as in engineering design and manufacturing? This makes us wonder, What is an Extended Entity-Relationship model? Before we jump in check out the previous part of this series to learn about what an Entity-relationship model is? Now, An Extended Entity-Relationship or EER model as the name suggests is a high-level data model that incorporates the extensions to the original ER model. They have the following three extensions: First, they include Subclasses and Superclasses which are the entities that belong to Generalization and specialization respectively. Put simply, Superclasses is an entity type that relates to one or more subclasses where Subclasses is a subgroup of entities with unique attributes. Second, is Union or Category type extension: It represents a single superclass or subclass relationship with more than one superclass. For example, while booking a car with a loan: Owner is a subset of the union of the three superclasses Company, Bank, and Person.
The third and final extension that an EER model offers is that of Aggregation. An ER diagram is not capable of representing the relationship between an entity and a relationship which may be required in some scenarios. In those cases, a relationship with its corresponding entities is aggregated into a higher level entity. For Example, Employee working for a project may require some machinery. So, REQUIRE relationship is needed between relationship WORKS_FOR and entity MACHINERY. Using aggregation, WORKS_FOR relationship with its entities EMPLOYEE and PROJECT is aggregated into a single entity and relationship REQUIRE is created between aggregated entity and MACHINERY. Hence, we first saw what an Extended Entity-Relationship model is and then went on to see what it's three extensions including Aggregation are?
In the next episode of MechanicaLEi find out what a relational model is?


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