ER Modeling Generalization, Specialization and Aggregation :

Generalization, Specialization and aggregation in DBMS are deliberation systems used to model data. ER-model has the power of expressing database entries in a conceptual hierarchical manner. As the basic concepts of ER modeling are often not enough to represent the requirements of the newer complex applications which therefore stimulated the need to develop additional ‘semantic’ modeling concepts. Various different semantic data models have been proposed and some of the most important semantic concepts have been successfully incorporated into the original ER model. The ER model supported with additional semantic concepts is called the Enhanced Entity-Relationship (EER) model.  There are three of the most important and useful added concepts of the EER model which are :

1. Generalization
2. Specialization
3. Aggregation

1. Generalization :

Generalization is a bottom-up approach in which two lower level entities combine to form a higher level entity. In generalization, a number of entities are accommodated together into one generalized entity or category based on their similar characteristics. In generalization, the higher level entity can also combine with other lower level entity to make further higher level entity. Generalization is used to emphasize the similarities among lower-level entity sets and to hide the differences in the schema. For example Saving account and Current account are all the bank Account.

2. Specialization : 

Specialization is opposite to Generalization. It is a top-down approach in which one higher level entity can be broken into two lower level entity. for example the specialization of student allows us to distinguish among students according to whether they are Ex-Student or Current Student. In specialization, some higher level entities mat not have lower-level entity sets at all. Specialization can be repeatedly applied to refine a design schema.

3. Aggregation : 

One limitation of the E-R model is that it cannot express relationships among relationships. To illustrate the need for such a construct, quaternary relationships are used which lead to redundancy in data storage. The best way to mode such situations is to use aggregation. Aggregation is a process when relation between two entity is treated as a single entity. For example the relation between Center and Course, is acting as an Entity in relation with visitor.

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