Introduction to ER Model

What is ER Model?

The Entity Relationship Diagram explains the relationship among the entities present in the database. ER models are used to model real-world objects like a person, a car, or a company and the relation between these real-world objects. In short, the ER Diagram is the structural format of the database.

Why Use ER Diagrams In DBMS?

• ER diagrams are used to represent the E-R model in a database, which makes them easy to convert into relations (tables).

• These diagrams are very easy to understand and easy to create even for a naive user.

• It gives a standard solution for visualizing the data logically.

Symbols Used in ER Model

Components of ER Diagram

ER Model consists of Entities, Attributes, and Relationships among Entities in a Database System.

Entity

An Entity may be an object with a physical existence – a particular person, car, house, or employee – or it may be an object with a conceptual existence – a company, a job, or a university course.

Entity Type

It refers to the category that a particular entity belongs to.

Example

• A table named student in a university database.
• A table named employee in a company database.

Entity Set

An entity set is a collection or set of all entities of a particular entity type at any point in time. The type of all the entities should be the same.

Example

• The collection of all the students from the student table at a particular instant of time is an example of an entity set.

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1. Strong Entity

A Strong Entity is a type of entity that has a key Attribute. Strong Entity does not depend on other Entity in the Schema. It has a primary key, that helps in identifying it uniquely, and it is represented by a rectangle. These are called Strong Entity Types.

1. Weak Entity

An entity that cannot be uniquely identified by its own attributes and relies on the relationship with other entity is called weak entity. it is represented by a double rectangle.

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Attributes

An entity is represented by a set of attributes, each entity has a value for each of its attributes.

Example
Student Entity has following attributes → Student_ID, Name, Course, Batch, Contact number

Types of Attributes

1. Simple Attribute

Attributes which can’t be divided further.

E.g.→ Customer’s account number in a bank, Student’s Roll number etc.

1. Composite Attribute

An attribute composed of many other attributes is called a composite attribute.

E.g. → Name of a person, can be divided into first-name, middle-name, last-name.

1. Single-valued Attribute

Only one value attribute.

E.g. → Student ID, loan-number for a loan.

1. Multi-valued Attribute

An attribute consisting of more than one value for a given entity.

E.g. → Phone_No (can be more than one for a given student).

1. Derived Attribute

An attribute that can be derived from other attributes is known as a derived attribute.

E.g. → Age (can be derived from DOB).

1. NULL Value Attribute

An attribute takes a null value when an entity does not have a value for it.

E.g. → person having no middle-name