Tuesday, 12 October 2021

Aggregation and composition in EER modeling

Relationships among relationships are not supported by the ER model. Groups of entities and relationships can be abstracted into higher level entities using aggregation.

Aggregation represents a “HAS-A” or “IS-PART-OF” relationship between entity types. One entity type is the whole, the other is the part. Aggregation allows us to indicate that a relationship set participates in another relationship set.

Consider the example of a driver driving a car. The car has various components like tires, doors, engine, seat, etc., which varies from one car to another. Relationship drives is insufficient to model the complexity of this system. Part-of relationships allow abstraction into higher level entities. In this example engine, tires, doors, and seats are aggregated into car.

A line with a diamond at the end is used to represent aggregation.


Composition is a stronger form of aggregation where the part cannot exist without its containing whole entity type and the part can only be part of one entity type.

Example-1: Consider the example of DEPARTMENT has PROJECT. Each project is associated with a particular DEPARTMENT. There cannot be a PROJECT without DEPARTMENT. Hence DEPARTMENT has PROJECT is an example of composition.

Composition is a form of aggregation that represents an association between entities, where there is a strong ownership between the as whole and the as part For example, a tree and a branch have a composition relationship. A branch is 'part' of a 'whole' tree - we cannot cut the branch and add it to another tree.

A line with a filled diamond at the end is used to represent composition


Example:The example of the Tree-Branch relationship can be represented as shown below:


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