EER diagrams are difficult to read when there are many entities and relationships. One possible solution is to group entities and relationships into entity clusters. Entity cluster is a set of one or more entity types and associated relationships grouped into a single abstract entity type.
Consider the example of Hospital Management. In hospital, the DOCTORS treat the PATIENT. The DOCTORS are paid by the MANAGEMENT which builds buildings. The DOCTORS can be either general physician or specialist like those with MS or MD. The patient can be either inpatient or outpatient. Here the DOCTOR specialization is clustered into DOCTORS entity and the PATIENT specialization is clustered into simply PATIENT. At the first glance, it may look like reduction of EER model to ER model, but it is not so. Here the entities as well as relationships are clustered into simply entity set.
Advantages of ER Modeling
An ER model is derived from business specifications. ER models separate the information required by a business from the activities performed within a business. Although business can change their activities, the type of information tends to remain constant. Therefore, the data structures also tend to be constant. The advantages of ER modeling are summarized later :
1.The ER modeling provides an easily understood pictorial map for the database design.
2.It is possible to represent the real world problems in a better manner in ER modeling.
3.The conversion of ER model to relational model is straightforward.
4.The enhanced ER model provides more flexibility in modeling real problems.
5.The symbols used to represent entity and relationships between entities are simple and easy to follow.