What do you mean by ‘Ternary Relationship’? Define the concept of aggregation with suitable example.
Ternary Relationship-
This is a relationship between three entity types. In the real world there are relationships other than those involving two things. For example, suppose that we want to capture which employees use which skills on which project. We might try to represent this data in a database as three binary relationships between skills and project, project and employee, and employee and skill. In order to capture the necessary information the database needs a ternary relationship. In this case the database needs a relationship, called used- on, among employee, skill, and project.
Aggregation- Aggregation is a process when relation between two entities is treated as a single entity.
Student attends the Course, and he has some subjects to study. At the same time, Course offers some subjects. Here a relation is defined on a relation. But ER diagram does not entertain such a relation. It supports mapping between entities, not between relations. So what can we do in this case?
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If we look at STUDENT and COURSE from SUBJECT’s point of view, it does not differentiate both of them. It offers it’s subject to both of them. So what can we do here is, merge STUDENT and COURSE as one entity. This process of merging is called aggregation. It is completely different from generalization. In generalization, we merge entities of same domain into one entity. In this case we merge related entities into one entity.
Here we have merged STUDENT and COURSE into one entity STUDENT_COURSE. This new entity forms the mapping with SUBJECTS. The new entity STUDENT_COURSE, in turn has two entities STUDENT and COURSE with ‘Attends’ relationship.
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