Generalization, Specialization, Aggregation
Last updated
Last updated
Generalization is like a bottom-up approach in which two or more entities of lower level combine to form a higher level entity if they have some attributes in common.
In generalization, an entity of a higher level can also combine with the entities of the lower level to form a further higher level entity.
Generalization is more like subclass and superclass system, but the only difference is the approach. Generalization uses the bottom-up approach.
In generalization, entities are combined to form a more generalized entity, i.e., subclasses are combined to make a superclass.
For example, Faculty and Student entities can be generalized and create a higher level entity Person.
Specialization is a top-down approach, and it is opposite to Generalization. In specialization, one higher level entity can be broken down into two lower level entities.
Specialization is used to identify the subset of an entity set that shares some distinguishing characteristics.
Normally, the superclass is defined first, the subclass and its related attributes are defined next, and relationship set are then added.
For example: In an Employee management system, EMPLOYEE entity can be specialized as TESTER or DEVELOPER based on what role they play in the company.
In aggregation, the relation between two entities is treated as a single entity. In aggregation, relationship with its corresponding entities is aggregated into a higher level entity.
For example: Center entity offers the Course entity act as a single entity in the relationship which is in a relationship with another entity visitor. In the real world, if a visitor visits a coaching center then he will never enquiry about the Course only or just about the Center instead he will ask the enquiry about both.
The database can be represented using the notations, and these notations can be reduced to a collection of tables.
In the database, every entity set or relationship set can be represented in tabular form.
The ER diagram is given below:
There are some points for converting the ER diagram to the table:
Entity type becomes a table.
In the given ER diagram, LECTURE, STUDENT, SUBJECT and COURSE forms individual tables.
All single-valued attribute becomes a column for the table.
In the STUDENT entity, STUDENT_NAME and STUDENT_ID form the column of STUDENT table. Similarly, COURSE_NAME and COURSE_ID form the column of COURSE table and so on.
A key attribute of the entity type represented by the primary key.
In the given ER diagram, COURSE_ID, STUDENT_ID, SUBJECT_ID, and LECTURE_ID are the key attribute of the entity.
The multivalued attribute is represented by a separate table.
In the student table, a hobby is a multivalued attribute. So it is not possible to represent multiple values in a single column of STUDENT table. Hence we create a table STUD_HOBBY with column name STUDENT_ID and HOBBY. Using both the column, we create a composite key.
Composite attribute represented by components.
In the given ER diagram, student address is a composite attribute. It contains CITY, PIN, DOOR#, STREET, and STATE. In the STUDENT table, these attributes can merge as an individual column.
Derived attributes are not considered in the table.
In the STUDENT table, Age is the derived attribute. It can be calculated at any point of time by calculating the difference between current date and Date of Birth.
Using these rules, you can convert the ER diagram to tables and columns and assign the mapping between the tables. Table structure for the given ER diagram is as below:
Figure: Table structure
