ER Diagram: What Is Entity Relationship Diagram With Examples

This is a comprehensive guide to the Entity-Relationship Diagram or ER Diagram including ER models, tools, symbols, examples and more:

Entity Relationship Diagram (ER Diagram or ERD) is a pictorial or visual representation of classifying groups or entities of common interest and defining the relationship between these groups. Hence, a structure is created with various symbols of different shapes and sizes so that it can be used as a model to depict the internal structure & relationship.

Entity Relationship Diagram is primarily used in the following areas:

  1. ER Models in Database Design: Used to visualize database tables and their relationships between them. Also finds usage in database troubleshooting.
  2. ER Diagrams in Software Engineering: Normally used in the planning stages of a software project. It is used for identifying different system elements and how they interact with each other. Also commonly known as Data Flow Diagrams (DFD).

What Is An Entity Relationship Diagram

ER Diagram

Entity Relationship Diagram (ERD) was a solution to the problem of not having a standard way to model databases or neither to business processes. Thus, it is a type of flowchart that helps us understand how different entities, objects, or concepts are related to each other within a system.

ER Diagram finds usage in fields such as business information system, software engineering, education & research, etc.

Here ERD or ER models are used through a defined set of symbols such as ovals, diamonds, rectangles, etc., and connecting lines to display the interconnectedness between them. ER Diagrams are also used in conjunction with Data Flow Diagrams (DFD), to show the flow of information within processes or systems.

What Is Entity Relationship Model

Entity-Relationship Model (ER Model) is a high-level data model or a graphical approach to database design, which defines the data elements and their relationship within a specified system or software.

ER Diagram is a way of representing ER Model graphically.

History of ER models:

Peter Chen is credited with the development of ER Modelling for database designs in the 1970s, through his 1976 seminal paper titled “The Entity-Relationship Model: Towards a Unified View of Data”.

Later around 1960–70s, Charles Bachman developed a type of Data Structure Diagram, which further contributed to the development of the Unified Modelling Language (UML), widely used in software design.

Suggested Reading =>> Data Modeling Tutorial

Why Use ER Diagrams

They find usage in some of these below key areas:

#1) Business Information Systems

The diagrams help in designing or analyzing relational databases used in the business processes. Any business process that has used fielded data involving entities can benefit from a relational database. It can be used for streamlining processes, and to uncover information more easily, and thus improve results.

#2) Database Troubleshooting

ER diagrams help to analyze existing databases and to identify and resolve problems in database logic or during deployment. Drawing them helps to highlight areas where things may have gone wrong.

#3) Business Process Re-engineering (BPR)

They are used to analyze databases used in business process re-engineering and also during modeling of a new database setup.

#4) Education

They are used in planning data structures for storing relational information and later retrieval from databases.

#5) Research

ER Diagrams are used in setting up databases to analyze structured data.

Components Of ER Diagram

It comprises:

  • Entity: Any object that can have data stored in it.
  • Relationships between entities: Defines how the entities are associated or related with each other.
  • Attributes of entities & relationships: Represents the characteristic or property of an entity.

ER Diagrams Symbols And Notations

#1) Entity

It is normally depicted as Rectangle.


Entity Type: A group of similar entities, which are having similar characteristics.

Entity Set: A particular sub-group within an entity type having a common sub-characteristic.

Entity Categories: It can be categorized as

  • Strong Entity
  • Weak Entity
  • Associative Entity

Difference between Strong and Weak Entity:

Strong EntityWeak Entity
It is not dependent on any entity.Dependent on a strong entity for existence.
It will always have a primary key.Does not have any primary key.
Represented by a single rectangle.Represented by a double rectangle.

Entity Keys: It is an attribute that helps to define an entity uniquely within an entity set.

Entity keys can be categorized as:

  • Super key: A set of attributes that helps together define an entity uniquely, within an entity set.
  • Primary or Candidate key: A key used to uniquely identify an entity set.
  • Foreign key: Helps to identify relationships between entities.

#2) Relationship

It is normally depicted as a diamond.


Difference between Strong and Weak Relationship:

Strong RelationshipWeak Relationship
A relationship, where one entity existence is not dependent on any entity.Dependent on a strong relationship for existence.
Contains primary key of the parent.Primary key is derived from another entity.
Represented by a single diamond.Represented by a double diamond.

#3) Attributes

It is normally depicted as an oval.


Difference between Key and Partial-key attribute:

Key attributePartial-key attribute
Denoted by the unique characteristics of the entity.Set of attributes to identify a weak entity set uniquely.
Unique in itself.It is partially unique, and combined with another strong entity.
Represented by a single oval with identifier.R Represented by a single oval with no identifier.

Other different attribute types:

#1) Simple attribute

  • Attributes that cannot be further divided.
  • Example: Aadhar number of any individual.

#2) Composite attribute

  • Constitutes of multiple simple attributes.
  • Example: A full name may consist of a combination of First Name & Last Name.

#3) Derived attribute

  • Attributes that do not directly exist in the database, but whose values are derived based on user requirements.
  • Example: Age can be derived based on DOB.

#4) Single-value attribute

  • Attributes that contain a single value.
  • Example: A field to store the Aadhar number of an individual.

#5) Multi-value attribute

  • Attributes that contain more than one value.
  • Example: Any individual may have more than one mobile no./email that are required to be stored.


It shows how an entity in one entity set is related to another entity within the same or other entity sets.

Below are some of the different types of cardinal relationships:

#1) One-to-One Relationships

  • One entity from an entity set is related to one entity in another entity set.
  • For example, A student can register for multiple courses. But for any particular course, it can be traced back to every single student.

#2) One-to-Many Relationships

  • One entity from an entity set is related to many entities in another entity set.
  • For example, A classroom with multiple students.

#3) Many to One Relationships

  • Many entities from an entity set are related to one entity in another entity set.
  • For example, many students belong to the same class.

#4) Many-to-Many Relationships

  • Many entities from an entity set are related to many entities in another entity set.
  • For example, many students associated with multiple faculty members and vice versa.

types of cardinal relationships

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Steps To Create An ER Diagram (ERD)

Below are some of the basic steps:


ER Diagram Examples

Entity Relationship Diagram, commonly known as ERD or ER Diagram or ER model, is one type of structural diagram used in database design. It contains different symbols and connectors that help to visualize two important information:

  1. The major entities present within the system scope
  2. The inter-relationships among these present entities.

Below is an example of the Loan system ER Diagram:

Entities: i) Borrower ii) Loan Request ii) Lender iv) Intermediary.

Relationship: How the Borrower Loan request is being captured by the lender, and the lending process takes place.


Below is an example of Subject wise Performance ER Diagram.

Entities: i) Students ii) Subject ii) Marks Obtained iv) Teacher.

Relationship: How the Students have performed on each Subject in terms of Marks allotted by the Teacher.


Advantages Of ERD

These are enlisted below:

  • Offers a visual representation of the overall structure.
  • Aids database designers to create an efficient design/architecture.
  • Helps to show the flow of data and the working of the entire system.
  • Acts as a blueprint for the existing database.
  • Aids in effective communication, as readers are able to understand relationships among different fields.
  • Provides flexibility in establishing and deriving relationships from the existing ones.
  • Good support for DBMS.

Disadvantages Of ERD

These are enlisted below:

  • Expression is limited.
  • Can be sometimes ambiguous.
  • It may not be always concise.
  • There are no industry-defined standards for documentation, so it may be confusing sometimes.
  • Hard to display information control.
  • Some data may be lost or covered up.

Importance Of Enhanced ER Diagram

These are enlisted below:

  • Enhanced Entity Relationship (EER) diagrams represent an expanded version of ER diagrams.
  • EER models are more helpful while designing databases with high-level models.
  • With more enhanced features, databases can be planned more efficiently by delving into the properties and constraints with much more precision.
  • It aids you in having a more detailed look at your information.
  • If the database would contain a larger amount of data, then it is advisable to switch to the enhanced model to gain a more deep understanding of your model.

Normalize An ER diagram

Application or provisioning of normalization leads to more robust requirement analysis, thus helping in overcoming some of the disadvantages.

Below are some of the reasons for the normalization of ERD:

  • Helps to improve the conceptual model.
  • Aids in speeding up implementation.
  • Helps in standardizing organizational vocabulary.
  • Enforce business rules.
  • Ensures adequate data quality.

Below are some of the basic ways/steps for normalization of ERD:

  • Transforming attributes into entity types.
  • Splitting compound attributes into further smaller attributes.
  • Expanding entity types into more two entity types and a relationship.
  • Transforming a weak entity type into a stronger entity type.
  • Adding historical details to an existing data model.

List Of The Online ER Tools

Multiple tools are available online for creating and editing ER models. Some of the prominent online tools are as mentioned below:

  • Pony ORM: One of the best SQL ORM used for fast prototyping.
  • Smartdraw: Easy and powerful for creating perfect layouts.
  • ERD Plus: Helps to draw with the step-by-step process explained for any user.
  • Lucidchart: Helps to create powerful visuals for every business need.
  • Creately: Helps in drawing the most complex data design easily with intuitive features.

Also Read =>> Open source Database Design Tools


ERD is one of the most used data modeling techniques. It helps in providing a framework for creating and manipulating databases. Thus, ERDs play a vital role in the Database designing approach and the entire process. It also acts as a common communication tool for technical and non-technical people.