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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