Extended E-R Features
An entity set can have subgroupings of entities that are different in some way or another from other entities in the set. An E-R model paths a way to represent these different entity groupings.
For example, a subset of entities in an entity set may have attributes that are not common among all the other entities in the entity set.
An entity set ‘Person’ can be further classified into:
- Employee b) Student
Each ‘Person’ type is depicted by a set of attributes that have all the attributes of an entity set ‘Person’, and may also contain additional attributes.
The entity ‘Student’ can have total_creds whereas the entity ‘Employee’ can have salary.
The naming of a subgroup in an entity is called as ‘Specialization’
A person can either be an Employee or Student, the entity Employee can further be classified into Manager and Clerks.
It is a containment relationship that is existing among a higher-level entity set and one or more lower-level entity sets.
In the above flowchart, every faculty and every clerk is an employee. Also, every Science and Commerce student is a student.
Therefore, The process of combining two or more classes into one class is called ‘GENERALIZATION’.
In the flowchart above,
Top-Down Approach is a SPECIALIZATION and Bottom-Up Approach is a GENERALIZATION.
What are Super Classes?
In the flowchart above, Person, Employee, Student are the Super classes
What are Subclasses?
Whereas Employee, Student, Faculty, Clerk, Science, Commerce are the Subclasses.
Extended E-R Feature