ER Model

What are ER Model ? Describe the advantages and dis-advantages of ER Model and what are the basic constructor of an ER Model, describe with diagram ?

◾ER model stands for Entity Relationship model. It is a high level data model. This model is used to define the data elements and relationship for the satisfied system.
◾It develops a conceptual design for the database. It also develops a very simple and easy to design view of data.
◾In ER model, the database structure is portrayed (तसवीर बनाना) as a diagram called a entity relationship diagram.
For Example : Suppose we design a school database, In this database, the student will be an entity with attributes like address, name, id, age etc.
◾The address can be another entity with attributes like city, street name, pin code etc and there will be a relationship between them.

Advantages of ER Model :-

◾Conceptually it is very simple.

◾Better visual representation.

◾Effective communication tool.

◾Highly integrated with Relational Model.

◾Easy conversion to any data Model

Dis-advantages of ER Model :-

◾Limited constraints and specifications.

◾Loss of information content.

◾Limited relationship representation.

◾No representation of data manipulation.

◾Popular for high level design.

◾No industry standard for notation

Components of ER Model :-

A. Entity :-

◾An entity may be any object, class, person or place. In the ER diagram, an entity can be represented as rectangles.
Example : Consider an organization as an example; Manager, Product, Employee, Department etc, can be taken as an entity.

1. Weak Entity :-

◾An entity that depends on another entity called a weak entity.
◾The weak entity doesn't contain any key attribute of its own.
◾The weak entity is represented by a double rectangle.

B. Attribute :-

◾The attribute is used to describe the property of an entity. Ellipse is used to represent an attribute.
For Example : i'd, age, address, number, name etc can be attribute of a student.

1. Key attribute :-

◾The key attributes is used to represent the main characteristics of an entity.
◾It represent a primary key.
◾The key attribute is represented by an eclipse with the text underlined.

2. Composite attribute :-

◾An attribute that composed of many other attributes is known as a composite attribute.
◾The composite attribute is represented by an ellipse and those ellipse are connected with an ellipse.

3. Multi valued Attributes :-

◾An attribute can have more than one value.
◾These attributes are known as a multi valued attributes.
◾The double oval is used to represent multi valued attribute.

Example : A student can have more than one phone numbers.

4. Derived Attribute :-

◾An attribute that can be derived from other attribute is known as a derived attribute.
◾It can be represented by a derived ellipse.
For example : A persons age changes over time and can be derived from another attribute like Date of birth.

C. Relationship :-

◾A relationship is used to describe the relation between entities.
◾Diamond is used to represent the relationship.

Read : What is relationship & Types of relationship

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