DIFFERENCES B/W Strong Entity Set and Weak Entity Set, DBMS and RDBMS,DBMS and FILE SYSTEM

Comparison between Strong Entity Set and Weak Entity Set is as follows:

Comparison between DBMS and RDBMS is as follows:

Comparison between DBMS and FILE SYSTEM is as follows:

Generalization,Specialization And Aggregration

The ER Model has the power of expressing database entities in a conceptual hierarchical manner. As the hierarchy goes up, it generalizes the view of entities, and as we go deep in the hierarchy, it gives us the detail of every entity included.
Going up in this structure is called generalization, where entities are clubbed together to represent a more generalized view. For example, a particular student named Mira can be generalized along with all the students. The entity shall be a student, and further, the student is a person. The reverse is called specialization where a person is a student, and that student is Mira.

Generalization

As mentioned above, the process of generalizing entities, where the generalized entities contain the properties of all the generalized entities, is called generalization. In generalization, a number of entities are brought together into one generalized entity based on their similar characteristics. For example, pigeon, house sparrow, crow and dove can all be generalized as Birds.

Specialization

Specialization is the opposite of generalization. In specialization, a group of entities is divided into sub-groups based on their characteristics. Take a group ‘Person’ for example. A person has name, date of birth, gender, etc. These properties are common in all persons, human beings. But in a company, persons can be identified as employee, employer, customer, or vendor, based on what role they play in the company.

Aggregration

Aggregration is a process when relation between two entity is treated as a single entity. Here the relation between Center and Course, is acting as an Entity in relation with Visitor.

E-R Diagram symbols and Notations

E-R Diagram

ER-Diagram is a visual representation of data that describes how data is related to each other.

 Symbols and Notations

Components of E-R Diagram

The E-R diagram has three main components.

1) Entity

An Entity can be any object, place, person or class. In E-R Diagram, an entity is represented using rectangles. Consider an example of an Organisation. Employee, Manager, Department, Product and many more can be taken as entities from an Organisation.

Weak Entity

Weak entity is an entity that depends on another entity. Weak entity doen't have key attribute of their own. Double rectangle represents weak entity.

2) Attribute

An Attribute describes a property or characterstic of an entity. For example, Name, Age, Address etc can be attributes of a Student. An attribute is represented using eclipse.

Key Attribute

Key attribute represents the main characterstic of an Entity. It is used to represent Primary key. Ellipse with underlying lines represent Key Attribute.

Composite Attribute

An attribute can also have their own attributes. These attributes are known as Composite attribute.

3) Relationship

A Relationship describes relations between entities. Relationship is represented using diamonds.

There are three types of relationship that exist between Entities.

• Binary Relationship
• Recursive Relationship
• Ternary Relationship

Binary Relationship

Binary Relationship means relation between two Entities. This is further divided into three types.
One to One : This type of relationship is rarely seen in real world.

The above example describes that one student can enroll only for one course and a course will also have only one Student. This is not what you will usually see in relationship.

One to Many : It reflects business rule that one entity is associated with many number of same entity. The example for this relation might sound a little weird, but this menas that one student can enroll to many courses, but one course will have one Student.

The arrows in the diagram describes that one student can enroll for only one course.

Many to One : It reflects business rule that many entities can be associated with just one entity. For example, Student enrolls for only one Course but a Course can have many Students.

Many to Many :

The above diagram represents that many students can enroll for more than one courses.

Recursive Relationship

Ternary Relationship

Relationship of degree three is called Ternary relationship