Notes

The graphical representation mechanism of a entity set having distinct subset of entities with shared set of attributes. consider a entity set Person with attributes Name , Emp_no , dept , Project_no. the person maybe further classified as 1) project member 2) project manager.

Each of this person types described by a set of attributes includes all attributes of the entity set Person + mode attributes. the specialization of a person allows us to distinguish amongst person according to whether they are project members or project manager

Generalization:
in the process of specialization there can be several attributes in common. the commonness is called generalization . in the above example Person is higher level entity set of project member or project manager. The entity set Person maybe defined as super class and the entity set project manager , project member can be defined as sub class. so generalization is simple inversion of specialization.

Aggregation:
it is an abstraction through which relationships are treated as higher level entities. the relationship set 'work' is related to the entity sets 'employee' , 'office' , and 'project' as a higher level entity set. such an entity set is treated in the same manner as any other entity set
a binary relationship 'managers' can be created between work and manager to represent who manages what tasks.