Specialization
An entity set may contain subgroupings of entities that are distinct in some way of other entities.
Such enetity set is a high-level entity set and can be brouken into one or more lower-level entity sets.
Course => undergraduate course / graduate course,
account=>saving account / checking account,
- An entity type E1 is a specialization of another entity type E2 if E1 has the same properties of E2 and perhaps even more.