entity relationship (er) diagrams assignment

A Guide to the Entity Relationship Diagram (ERD)

Learn all about Entity Relationship Diagrams (ERDs), what they are used for, how to understand them, how to create them, and more in this guide.

Table of Contents

What is an Entity Relationship Diagram (ERD)?

An Entity Relationship Diagram (ERD) is a type of diagram that lets you see how different entities (e.g. people, customers, or other objects) relate to each other in an application or a database.

They are created when a new system is being designed so that the development team can understand how to structure the database. They can also be created on an existing system to help the team understand how the system works and to find and resolve any issues.

Entity Relationship Diagrams use a specific set of symbols, such as shapes and arrows, to depict the system and database.

Here’s an example of an ERD:

Components of an ERD

An Entity Relationship Diagram is made up of many different components:

Relationship

An entity is a thing that can have data stored about it. It can be a physical object (e.g. car, person), a concept (e.g. address) or an event (e.g. student enrolment in a course). They represent nouns.

They are usually represented as rectangles on an ERD with the entity name inside the rectangle.

An entity can also be a strong entity or a weak entity. What’s the difference?

A strong entity has an identifier (a primary key) and does not depend on any other entities for it to exist. For example, a student may be a strong entity, as it can have a primary key and does not depend on any other entities for it to exist.

A weak entity is one that depends on a strong entity for existence. This means it has a foreign key to another entity. For example, an enrolment of a student may be a weak entity, as an enrolment cannot exist without a student.

A relationship in an ERD defines how two entities are related to each other. They can be derived from verbs when speaking about a database or a set of entities.

Relationships in ERDs are represented as lines between two entities, and often have a label on the line to further describe the relationship (such as “enrols”, “registers”, “completes”).

There are several types of relationships that are represented on an ERD:

• One to one: One record of an entity is directly related to another record of an entity
• One to many: One record of an entity is related to one or more records of another entity.
• Many to many: Many records of one entity can be related to many records of another entity.

An attribute is a property of an entity or something that can be used to describe an entity. They are often represented as ovals, or as entries inside an entity.

There are several different types of attributes represented on an ERD:

• Simple: an attribute that cannot be split into other attributes, such as a first name.
• Composite: an attribute that can be split into other attributes, such as name being split into first, middle, and last name.
• Derived: an attribute that is calculated or determined from another attribute, such as the age of record being calculated from the created date.

An attribute can also be single-value or multi-value:

• Single-value: an attribute that is only captured once for
• Multi-Value: an attribute that can be captured more than once for an entity, such as multiple phone numbers.

What is Cardinality?

Cardinality represents the number of instances of an entity that exist in a relationship between two entities. This is often expressed as a number but could also be a symbol, depending on the style of diagram used. Common cardinality values are zero, one, or many.

We’ll see some examples of cardinality later in this guide.

Natural Language

When we are creating an ERD we often have an idea of what we want to capture. This can usually be expressed in words, or using “natural language”.

Some examples are:

• “Record students, courses they enrol in, and teachers who teach the course”
• “Capture the customer orders, customer details, and where the orders are being sent”
• “Capture patient data and the operations they had”

These sentences include a few different types of words, which can be used as a starting point for an ERD. They are represented in a few different ways:

• Noun: a “thing”, such as a student or customer. Represented as an entity.
• Verb: an action, such as enrol or send. Represented as a relationship between two entities.
• Adjective: a describing word, such as residential or advanced. Represented as an attribute on an entity.

This can help you translate a description of what you need to diagram into an actual diagram.

Symbols and notations

When creating an ERD, it can be simple to create boxes and lines between them. But, like many things in software development, there are a few different methods and standards that are available. For ERDs, there are several notation standards, which define the symbols used.

Here’s an example of the symbols used with the Chen notation style.

Here’s an example ERD using Chen notation:

Crow’s Foot

Here’s an example of the symbols used with the Crow’s foot notation style. This is the style you’ll see the most on Database Star as it’s the one I’m most familiar with. It’s called “crow’s foot’ as the symbol for a many relationship looks like the foot of a crow with its three prongs.

Here’s an example ERD using Crow’s Foot notation:

Here’s an example of the symbols used with the Bachman notation style.

Here’s an example ERD using Bachman notation:

Here’s an example of the symbols used with the IDEF1X notation style.

Here’s an example ERD using IDEF1X notation:

Here’s an example of the symbols used with the Barker notation style.

Conceptual, logical, physical

An Entity Relationship Diagram can be drawn at three different levels: conceptual, logical, or physical.

Each of these levels has a different level of detail and are used for a different purpose.

Let’s see some examples.

Conceptual Data Model

The conceptual data model shows the business objects that exist in the system and how they relate to each other.

It defines the entities that exist, which are not necessarily tables. Thinking of tables is too detailed for this type of data model.

An example of a conceptual data model is shown here. It shows students, courses, and how they relate.

Logical Data Model

A logical model is a more detailed version of a conceptual data model. Attributes are added to each entity, and further entities can be added that represent areas to capture data in the system.

Here’s an example of the student and course data model created as a logical data model.

Physical Data Model

The physical data model is the most detailed data model in this process. It defines a set of tables and columns and how they relate to each other. It includes primary and foreign keys, as well as the data types for each column.

These diagrams can be created manually in a data modelling tool. They are also often generated by an IDE from an existing database.

Here’s an example of the student and course physical data model.

This table outlines the differences between a conceptual, logical, and physical model:

How to Create an Entity Relationship Diagram

So how do you create a data model or an Entity Relationship Diagram?

I’ve detailed this in my Relational Database Design course, and I’ve also explained it in my guide to designing a database .

The process to create an ERD is:

• Write a sentence or two about what you are storing data about
• List what you’re storing data about – the nouns/objects
• List the information you want to store for each object
• Describe the relationships between each object
• Draw the diagram

The steps listed here are brief but the process can take some time, depending on how familiar you are with the system and how experienced you are with creating Entity Relationship Diagrams.

Tools to Use

What tool should you use to create a data model or an Entity Relationship Diagram?

There are a lot of applications available to help you create one. I’ve written a post that lists them all with a comparison of major features and price, which you can find here: 79 Data Modeling Tools Compared .

Personally, I use LucidChart as it’s simple to use and web-based. But in the past, I’ve used Visio and even the data modelling capabilities inside an IDE.

That page above includes a range of tools and some recommendations if you’re not sure what to use.

You can also start with a pen and paper to get an understanding of how a diagram can be created. I often start by drawing one in my notebook. It gets messy eventually, but I transfer it to an electronic copy anyway.

Tips for Creating an Entity Relationship Diagram

Here are some tips for creating an Entity Relationship Diagram:

• Determine the right level of detail based on the purpose of the diagram. Development teams often find the logical model the most useful, but others might find the conceptual model more valuable.
• Review the entities and attributes to see if they include everything you want to store in the system.
• Name all entities and attributes.
• If you have a large diagram, consider using colours to highlight different sections, or breaking it up into smaller diagrams.
• Be consistent with your naming and the symbols you use.

An Entity Relationship Diagram is a great tool to help you define, understand, and communicate the requirements of a system. It can be modelled at a high level (conceptual data model), a detailed level (physical data model) or a level in between (logical data model).

There is a range of modelling notations or types of symbols which define how entities, attributes, and relationships are captured.

If you want to get started with creating an ERD for your system, consider drawing one on paper, or use one of the many tools available.

19 thoughts on “A Guide to the Entity Relationship Diagram (ERD)”

Hi, may I congratulate you on a job very well done. Your comparative diagram notation are clear, you don’t use too much jargon. Kudo

Very clearly explained, Thank You

excellent job with the explanation. Thanks, it really helps👍👍

Great insight will be coming can you help me solve my question from my class? To identify a problem of an organization and develop ER-Diagram and a database management system to solve it.

You need to give lecture to our doctor at the university. I am glad that English is the science language not German or any other language,

Very well explained, as I was struggling of what is a ERD, how do I design it. But now you have explained clearly as I understand very much. I like to say thank you for your hard effort.

Nice post!I will read more post in this site! Thank you!

Great article, really clear and well-written -thank you!

Just one minor point; I think the second Bachman diagram should actually be a Barker example?

Dear mister genius, thank you.

The notes are very clear and understandable.

i support you dear.

Thats the best description of data modelling I’ve seen in my quest to understand the concept

Excellent Explain

help me a lots, tyvm

Very understandable 😊

Great job, thank you for explaning so clearly

thank you so mach

this is great content manhz, very simple but detailed explanation init

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How to Draw Entity Relationship Diagrams (ERDs)

Entity Relationship Diagrams serve as valuable tools for designing complex systems and their relationships.

This article will go through the step-by-step process of designing an ER diagram and defining how entities, attributes, and relationships are defined. Entity relationship diagrams are important in database design and require a clear structure for all data.

What is an Entity Relationship Diagram?

An Entity Relationship Diagram (ERD) is a graphical representation that shows how entities, such as people, objects, or concepts, relate to each other within a system.

ERDs are commonly used in database design to visualize the relationships between entities and their attributes.

They help in understanding the logical structure of databases by showing entities' connections and relationships using symbols like rectangles, diamonds, ovals, and connecting lines.

ERDs are essential for designing or debugging relational databases in software engineering, business information systems, education, and research.

Need to Draw ER Diagram

ER diagram or ER model, is drawn to visually represent the relationships between entities in a database system. Some benefits of drawing ER diagrams for database design are:

• It helps in understanding the data relationships.
• ER diagrams are like a blueprint for designing a database.
• Helps in communicating about databases with database designers, developers, users, etc.
• They help in describing different relationships and operations within an organization.

How to Draw an Entity Relation Diagram (ERD)

A step-by-step process to draw an entity relation diagram (ERD) is:

Step 1: Identifying Entities

Determine the main objects you want to represent in the database. Eg, "students", "courses", or "products".

Step 2: Defining Attributes

Identify the properties(attributes) of properties of each entity. These attributes provide more details about an entity.

Step 3: Specifing Relationships

Create relationships between entities to specify how entities interact with each other. Relationships are verbs like "teaches", "studies", or "sells".

Step 4: Drawing Entities

Draw entities as rectangle and write the name.

Step 5: Adding Attributes

To add attributes of a entitity write attributes inside the rectangle or connect them with lines.

Step 6: Connecting Entities

Draw lines between the related entities to represent their connection.

Step 7: Specifying Cardinality

Indicate the minimum and maximum number of relationship instances associated with an entity using notations like crow's foot.

Step 8: Organizing ER Diagram

Organize all entities and relationships in a clean way for better readibility and understanding.

Draw Entity Relationship Diagram Example

After learning the steps of how to draw an enitity relationship diagram, we will create a demo ER diagram.

Let’s take an example of ER diagram for a bank through which we can learn how to design an ER and understand all the required methods.

Entity Relationship Diagram for BANK

We will follow the steps mentioned above, to draw entity relationship diagram for bank.

Defining Entities

A thing in the real world with an independent existence. It is may be an object with physical existence (ex : house, person) or with a conceptual existence (ex: course, job ). The are represented by rectangle .

Entities for Bank are:

Bank, Branch, Employee, customer, loan, account.

Adding Attributes

Attributes are the kind of properties that describe the entities. They are represented by ovals .

Attributes for Bank are:

• For Bank Entity the Attributes are Bname, code.
• For Branch Entity the Attributes are Blocation, Bname.
• For Employee Entity the Attributes are Eid, Designation, salary.
• For Customer Entity the Attributes are Cid, Cname, Address, DOB.
• For Loan Entity the Attributes are Loan_no, amount, rate.
• For Account Entity the Attributes are acc_no, type.

Establishing Relationships

Entities have some relationships with each other. Relationships define how entities are associated with each other .

Let's Establishing Relationships between them are:

• The Bank has branches .
• The Branch provides loan .
• The Employee works in branch .
• The Branch contains customers .
• The Customers has account .
• The Branch maintains account .
• The Customer avails loan .

Specifying Cardinality

Cardinality defines the numerical constraints on the relationships between entities. It is a notation that tells the ERD reader whether there are one , many or some combination of those factors between each entity.

1. One to One relationship(1:1)

In the entity relationship diagram, the one to one relationship means that single entity in one table is associated with single entity in another table . For example, one driver have only one license .

2. One to Many relationship(1:N)

One to many relationships means that single entity in one table is related to more than one entities in another table. For example, one bank has many branches .

3. Many to One(N:1)

Many-to-one relationship in entity relationship diagram means that multiple entities are related to only single entity in another table. For example, many developers works on single project.

4. Many to Many relationship(M:N)

Many to many relationship means that multiple entities in one table is associated with multiple entities in another table. For example, multiple customers have multiple accounts.

Specify cardinality for Bank:

• Bank and branch has One to Many relationship (a bank has multiple branches).
• Branch and loan has also One to Many relationship(a branch can provide many loans).
• Branch and employee has One to Many relationship(one branch has many employees).
• Branch and account has One to Many relationship(one branch has many accounts).
• Branch and customer has Many to Many relationship(multiple branches have multiple customers).
• Customer and account has Many to Many relationship(multiple customers have multiple accounts).
• Customer and loan has Many to Many relationships(multiple customers have multiple loans)

Identify Primary Keys

Primary keys are the unique identifier for each record in database table. It is denoted by an underline under the attribute name.

• The Primary key of Bank is code .
• The Primary key of Branch is branch_code.
• The Primary key of Employee is Eid.
• The Primary key of Customer is Cid.
• The Primary key of Loan is loan_no .
• The Primary key of Account is acc_no .

Final ER Diagram

The below diagram is our final entity relationship diagram for bank with all entities , their attributes and the relationship between them with the PRIMARY KEY and Cardinality ratio .

Benefits of an ER Diagram

• ER diagrams provides a clear and visual representation of the database structure. This make it easier for developers and designers to understand the relationships between the entities.
• It is an effective communication tool for database designers and the stakeholders which helps to understand the data model.
• ER diagrams serve as the foundation for designing the database schema. They guide the creation of tables, relationships and constraints in the database management system.
• Creating an ER diagram helps in understanding and analyzing the data requirements of the system.
• Database managers can more quickly and efficiently undertake maintenance activities, including adding new entities or changing existing relationships, when they have a well-designed Entity Relationship diagram that makes the database structure easy to understand.

This article covers a step-by-step guide on drawing an ER diagram, focusing on the significance of accurate representation, clear relationships, and well-defined attributes.

It covers the meaning, need, benefits, and an example of an Entity Relationship Diagram (ERD). The provided example showcases the design process for a bank's database, including identifying relationships, cardinality, and primary keys. The ER diagram serves as a visual representation of the database structure, with room for refinement based on specific requirements or stakeholder feedback.

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What is an Entity Relationship Diagram (ERD)? | An Introduction to ER Diagram

So you want to learn Entity Relationship diagrams? This ER diagram tutorial will cover their usage, history, symbols, notations and how to use our ER diagram software to draw them. We’ve also added some templates for you to get started quickly.

What is an ER diagram?

An Entity Relationship Diagram (ERD) is a visual representation of different entities within a system and how they relate to each other . It is a tool used to design and model relational databases, and shows the logical structure of the database. ER diagrams use symbols to represent entities, attributes, and relationships, which help to illustrate the relationships between the entities in the database. It consists of entities (which represent objects or concepts, like “Customer” or “Product”) and relationships (which represent how these entities are connected or interact with each other, like “owns” or “buys”).

Entities are depicted as rectangles, and relationships are shown as diamonds. Lines connecting entities and relationships represent the connections between them, often indicating the type of relationship (such as one-to-one, one-to-many, or many-to-many).

ER diagrams are commonly used in software engineering and database design to help developers and stakeholders understand and design complex databases. For example, the elements writer, novel, and a consumer may be described using ER diagrams the following way:

• Ready to use
• Fully customizable template
• Get Started in seconds

History of ER Diagrams

ER diagrams were first introduced by a computer scientist named Peter Chen in 1976. He proposed them as a way to visually represent the structure of databases. During the late 1970s and early 1980s, ER diagrams became widely used in both academic and industrial settings. They provided a clear way to show how different pieces of information are related to each other.

As database systems became more complex, ER diagrams remained important tools for designing and managing databases. They helped developers and users understand how data was organized and connected.

In the 1990s and 2000s, with the rise of the internet and new technologies, ER diagrams adapted to accommodate these changes. They continued to be essential for designing databases, but they were sometimes used alongside other modeling techniques.

Today, ER diagrams are still widely used in software engineering and database design. They provide a simple and intuitive way to plan and communicate the structure of databases, making them valuable tools for developers and designers.

ER Diagram Templates

Below are some ER diagram templates so you can get started quickly. Clicking on the image and in the new page that opens click the “Use as Template” button. For more templates check our ER diagram template section .

Uses of ER Diagrams?

ER diagrams are essential tools for designing, communicating, maintaining, and optimizing databases. They facilitate collaboration, ensure data integrity, and contribute to the efficient management of database systems. Here are several uses of ER diagrams.

Database design: ER diagrams are used to design databases. They provide a visual representation of how different pieces of information, called entities, are related to each other. This helps in planning the structure of the database before it’s built.

Communication: ER diagrams act as a common language between people involved in developing databases, such as developers, designers, and clients. They help everyone understand how the database will work and how data will be organized.

Database maintenance: Even after a database is created, ER diagrams remain useful for maintenance. They help developers understand the existing structure and make changes or updates as needed while ensuring the integrity of the database.

Documentation: ER diagrams serve as documentation for the database system. They provide a detailed overview of the database schema, including entities, attributes, and relationships. This documentation helps in troubleshooting issues and training new team members.

Identifying redundancy and inconsistencies: By visualizing relationships between entities, ER diagrams help identify redundant or inconsistent data in the database design. This allows for optimization and ensures data accuracy.

Database optimization: ER diagrams aid in optimizing database performance. By understanding entity relationships, developers can design efficient queries and data retrieval strategies, leading to improved performance and scalability.

Integration with development tools: ER diagrams can be integrated into various development tools and database management systems. Many software tools provide features for creating, editing, and generating code from ER diagrams, streamlining the database development process.

The Components and Features of ER Diagrams

An entity represents a real-world object, concept, or thing about which data is stored in the database. It could be a person, place, thing, event, or concept.

Characteristics

Attributes: Entities have attributes that describe their properties or characteristics. Instances: Each entity has instances or occurrences, representing individual occurrences of the entity in the real world. Identity: Each instance of an entity is uniquely identifiable and can be distinguished from other instances. Relationships: Entities can be related to other entities through relationships, forming connections within the database schema.

Types of entities

Strong entities: Also known as regular entities, these entities have attributes that uniquely identify each instance. They can exist independently of other entities. Weak entities: Weak entities do not have a primary key attribute of their own and depend on a related strong entity (owner entity) for their existence. They are identified by their relationship with the owner entity. Associative entities: Also called many-to-many entities, these entities are used to represent relationships between relationships. They arise in many-to-many relationships and typically contain attributes in addition to the foreign keys linking the associated entities.

Types of entity keys

Simple key: A single attribute serves as the primary key. Composite key: A combination of two or more attributes serves as the primary key. It is used when no single attribute uniquely identifies each instance. Surrogate key: An artificially generated identifier, such as an auto-incremented integer, is used as the primary key. It is often added when no suitable natural key exists or when there are concerns about stability or size of natural keys.

Attributes are the characteristics or properties of entities. They describe the details of each entity. For example, attributes of a “Customer” entity might include “Customer ID,” “Name,” and “Email Address.”

Name: Each attribute has a name that identifies it within the entity. Data type: Attributes have a data type that specifies the kind of data they can hold, such as text, number, date, etc. Constraints: Attributes may have constraints that define rules or conditions for the values they can contain, such as uniqueness or requiredness. Domain: Attributes have a domain, which is the set of all possible values that the attribute can take. Nullability: Attributes may allow null values, meaning they can have no value assigned to them for some instances of the entity.

Types of attributes

Simple attribute: A single atomic value is stored in the attribute. Example: Name, Age. Composite attribute: An attribute that can be divided into smaller sub-parts, each representing a distinct attribute. Example: Address (composed of Street, City, Zip Code). Single-valued attribute: An attribute that holds a single value for each instance of the entity. Multi-valued attribute: An attribute that can hold multiple values for each instance of the entity. Example: Phone Numbers.

Relationships

Relationships show how entities are connected or related to each other in the database. They represent interactions or associations between entities. Examples include “owns,” “buys,” or “works for.”

Name: Each relationship has a name that describes the nature of the association between entities. Directionality: Relationships can be unidirectional or bidirectional, indicating the flow or direction of the association between entities. Cardinality: Cardinality specifies the number of instances of one entity that can be associated with instances of another entity in a relationship. It defines the minimum and maximum occurrences of related entities. Degree: The degree of a relationship indicates the number of entities participating in the relationship. Relationships can be unary (involving a single entity), binary (involving two entities), ternary (involving three entities), etc. Attributes: Relationships can have attributes that describe additional information or properties about the association between entities. Recursive relationships: Some relationships involve an entity being related to itself, known as recursive relationships. They represent connections or interactions within the same entity.

Types of relationships

One-to-one (1:1): Each instance of one entity is associated with exactly one instance of another entity. One-to-many (1:N): Each instance of one entity is associated with multiple instances of another entity, but each instance of the other entity is associated with only one instance of the first entity. Many-to-one (N:1): Multiple instances of one entity are associated with exactly one instance of another entity. Many-to-many (M:N): Multiple instances of one entity are associated with multiple instances of another entity, and vice versa. Recursive relationships: An entity is related to itself through a relationship. This can represent hierarchical structures or self-referential associations.

Cardinality

Cardinality specifies the number of instances of one entity that can be associated with the number of instances of another entity in a relationship. It describes the minimum and maximum number of occurrences of one entity that can be associated with occurrences of another entity.

Minimum cardinality: Specifies the minimum number of instances required in a relationship. Maximum cardinality: Indicates the maximum number of instances allowed in a relationship. Directionality: Determines if the relationship is one-way or two-way. Completeness: Shows whether participation in the relationship is mandatory or optional. Multiplicity: Defines the range of valid occurrences for each entity. Constraint enforcement: Ensures data integrity by enforcing rules on entity participation.

Entity Relationship Diagram (ERD) Symbols and Notations

There are three basic elements in an ER Diagram: entity, attribute, relationship. There are more elements which are based on the main elements. They are weak entity, multi valued attribute, derived attribute, weak relationship, and recursive relationship. Cardinality and ordinality are two other notations used in ER diagrams to further define relationships.

An entity can be a person, place, event, or object that is relevant to a given system. For example, a school system may include students, teachers, major courses, subjects, fees, and other items. Entities are represented in ER diagrams by a rectangle and named using singular nouns.

Weak Entity

A weak entity is an entity that depends on the existence of another entity. In more technical terms it can be defined as an entity that cannot be identified by its own attributes. It uses a foreign key combined with its attributed to form the primary key. An entity like order item is a good example for this. The order item will be meaningless without an order so it depends on the existence of the order.

An attribute is a property, trait, or characteristic of an entity, relationship, or another attribute. For example, the attribute Inventory Item Name is an attribute of the entity Inventory Item. An entity can have as many attributes as necessary. Meanwhile, attributes can also have their own specific attributes. For example, the attribute “customer address” can have the attributes number, street, city, and state. These are called composite attributes. Note that some top level ER diagrams do not show attributes for the sake of simplicity. In those that do, however, attributes are represented by oval shapes.

Multivalued Attribute

If an attribute can have more than one value it is called a multi valued attribute. It is important to note that this is different from an attribute having its own attributes. For example, a teacher entity can have multiple subject values.

Derived Attribute

An attribute based on another attribute. This is found rarely in ER diagrams. For example, for a circle, the area can be derived from the radius.

Relationship

A relationship describes how entities interact. For example, the entity “Carpenter” may be related to the entity “table” by the relationship “builds” or “makes”. Relationships are represented by diamond shapes and are labeled using verbs.

Recursive Relationship

If the same entity participates more than once in a relationship it is known as a recursive relationship. In the below example an employee can be a supervisor and be supervised, so there is a recursive relationship.

Cardinality and Ordinality

These two further defines relationships between entities by placing the relationship in the context of numbers. In an email system, for example, one account can have multiple contacts. The relationship, in this case, follows a “one to many” model. There are a number of notations used to present cardinality in ER diagrams. Chen, UML, Crow’s foot, Bachman are some of the popular notations. Creately supports Chen, UML and Crow’s foot notations.The following example uses UML to show cardinality.

How to Draw ER Diagrams

Below points show how to go about creating an ER diagram.

• Identify all the entities in the system. An entity should appear only once in a particular diagram. Create rectangles for all entities and name them properly.
• Identify relationships between entities. Connect them using a line and add a diamond in the middle describing the relationship.
• Add attributes for entities. Give meaningful attribute names so they can be understood easily.

Sounds simple right? In a complex system, it can be a nightmare to identify relationships. This is something you’ll perfect only with practice.

Conceptual, Logical and Physical Data Models

Conceptual, logical, and physical data models are three distinct stages in the process of designing and implementing a database system. Each model serves a specific purpose and targets different levels of abstraction.

Conceptual Data Model

Purpose: The conceptual data model represents the highest level of abstraction and focuses on understanding the business requirements and concepts without considering implementation details. Characteristics: Describes entities, their attributes, and the relationships between them at a conceptual level. Independent of any specific database management system (DBMS) or technology. Provides a global view of the entire database system without delving into technical details. Representation: Often depicted using Entity-Relationship (ER) diagrams or similar graphical tools. Example: In a university system, the conceptual data model might include entities like Student, Course, and Instructor, along with their attributes and relationships.

Logical Data Model

Purpose: The logical data model translates the conceptual model into a more detailed and structured representation that can be implemented in a specific DBMS. Characteristics: Defines the structure of the database using tables, columns, keys, and relationships. Captures data requirements and relationships more precisely than the conceptual model. Still independent of the physical implementation details such as storage optimizations or indexing strategies. Representation: Often depicted using Entity-Relationship (ER) diagrams or Unified Modeling Language (UML) class diagrams, along with normalization techniques. Example: In the university system, the logical data model would specify tables like Student, Course, and Instructor, along with their attributes and primary/foreign keys.

Physical Data Model

Purpose: The physical data model represents the actual implementation of the database design in a specific DBMS, considering storage structures, indexing strategies, and optimization techniques. Characteristics: Translates the logical data model into a schema that can be directly implemented in a DBMS. Specifies details such as data types, indexes, partitions, and storage allocation. Optimized for performance, scalability, and efficiency based on the requirements of the application and the capabilities of the chosen DBMS. Representation: Often represented using Data Definition Language (DDL) scripts or database schema diagrams specific to the chosen DBMS. Example: In the university system, the physical data model would include details like table definitions with specific data types, indexes on frequently queried columns, and partitioning strategies for large tables.

Limitations of ER Diagrams

While Entity-Relationship diagrams are widely used for database design and modeling, they do have some limitations:

ER diagrams can get really complicated, especially in big systems with lots of entities and relationships. It can be tough to manage and understand these complicated diagrams, making it harder to see how the database works overall.

Sometimes ER diagrams don’t capture everything about the real world accurately. This can happen if the descriptions used to make the diagrams aren’t clear or if there are differences in how people understand things. It can lead to mistakes or confusion in the diagram.

ER diagrams focus on the big picture of the database structure, but they might miss out on some of the smaller details, like how the data is actually stored or used. This means they might not show everything about how the database works.

ER diagrams have trouble showing certain kinds of relationships or rules well. For example, really complicated structures or rules about time or space can be hard to draw accurately using ER diagrams.

ER diagrams might not be great at showing certain ideas from object-oriented programming, like inheritance or polymorphism. It can be tricky to translate these ideas into ER diagrams, and it might need extra explanations or changes to make them work.

As databases change over time, ER diagrams might need to be updated to show the changes accurately. Keeping track of these changes and making sure the diagrams match the actual database can take a lot of time and effort.

ER Diagram Best Practices

Provide a precise and appropriate name for each entity, attribute, and relationship in the diagram. Terms that are simple and familiar always beats vague, technical-sounding words. In naming entities, remember to use singular nouns. However, adjectives may be used to distinguish entities belonging to the same class (part-time employee and full-time employee, for example). Meanwhile attribute names must be meaningful, unique, system-independent, and easily understandable.

Remove vague, redundant or unnecessary relationships between entities.

Never connect a relationship to another relationship.

Make effective use of colors. You can use colors to classify similar entities or to highlight key areas in your diagrams.

Benefits of ER diagrams

ER diagrams constitute a very useful framework for creating and manipulating databases. First, ER diagrams are easy to understand and do not require a person to undergo extensive training to be able to work with it efficiently and accurately. This means that designers can use ER diagrams to easily communicate with developers, customers, and end users, regardless of their IT proficiency.

Second, ER diagrams are readily translatable into relational tables which can be used to quickly build databases. In addition, ER diagrams can directly be used by database developers as the blueprint for implementing data in specific software applications.

Lastly, ER diagrams may be applied in other contexts such as describing the different relationships and operations within an organization.

Drawing ER Diagrams Using Creately

You can draw entity relationship diagrams manually, especially when you are just informally showing simple systems to your peers. However, for more complex systems and for external audiences, you need diagramming software such as Creately to craft visually engaging and precise ER diagrams. The ER diagram software offered by Creately as an online service is pretty easy to use and is a lot more affordable than purchasing licensed software. It is also perfectly suited for development teams because of its strong support for collaboration.

FAQs about Entity Relationship Diagrams

Why are er diagrams important, what are the common mistakes to avoid when creating er diagrams.

• Not defining the entities and relationships clearly
• Overcomplicating the diagram by including too many entities and relationships
• Using inconsistent naming conventions for entities and attributes
• Failing to specify cardinality and participation constraints for relationships
• Using unclear or ambiguous symbols or terminology

What types of relationships can be represented in an ER diagram?

ER diagrams can represent several types of relationships, such as:

• one-to-one,
• one-to-many, and
• many-to-many relationships.

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ER Diagrams: The Complete Guide to Database Modeling

8 minutes read

What is an ER Diagram?

An Entity-Relationship Diagram (ER diagram) is a type of data modeling technique that visually illustrates an information system's entities and the relationships between those entities. It's used in database design to organize complex systems by demonstrating how things interact with one another within the system.

In an ER Diagram, entities are depicted as rectangles, while relationships are shown as diamonds that connect entities. Moreover, the properties of these entities, also known as attributes, are depicted as ovals. Each element in the diagram serves a purpose in understanding the database's structure, thereby enabling efficient database management and accurate communication of system processes.

ER Diagrams serve as the blueprint for constructing databases, and they can represent real-world scenarios in an uncomplicated yet detailed manner. They aid in determining what type of information to store, how data elements relate to one another, and where to implement rules to ensure data accuracy and consistency.

In a nutshell, ER Diagrams are instrumental in facilitating a structured and systematic approach towards database design, making them essential tools in fields like software engineering, systems analysis, and information systems.

What are the Types of ER Diagrams?

ER Diagrams, short for Entity-Relationship Diagrams, are used in database design to illustrate the logical structure of databases. There are three primary types of ER Diagrams: Conceptual ER Diagrams, Logical ER Diagrams, and Physical ER Diagrams.

1. Conceptual ER Diagrams

Conceptual ER Diagrams, also known as high-level ER diagrams or conceptual data models, depict a high-level view of an organization's data requirements without going into detailed specifics. They mainly identify entities and relationships without defining the attributes of these entities. These diagrams are typically used in the initial stages of system development to define the scope and general contents of organizational databases.

2. Logical ER Diagrams

Logical ER Diagrams, or logical data models, delve deeper than the conceptual model to define the specific attributes of entities and their relationships. This diagram specifies primary keys for each entity and describes how they interact through relationships. However, it doesn't detail how these elements will be implemented physically within the database system. It's often used to structure the database's design further before moving into the physical modeling phase.

3. Physical ER Diagrams

Physical ER Diagrams, also known as physical data models, take the specifications detailed in the logical ER diagrams and translate them into physical components that make up a database system. They include all table structures, primary and foreign keys, stored procedures, triggers, indexes, and other details that will be physically implemented in the database system. They are usually created by database designers who are familiar with the specific software being used for database implementation.

By understanding these three types of ER diagrams, you can better understand and participate in various phases of database modeling and design.

When to Use an ER Diagram?

Entity Relationship diagrams are a versatile tool that can be beneficial in a variety of scenarios:

• Database Design

ER Diagrams are predominantly used during the design phase of a database. They help in visualizing the different entities, relationships, and attributes involved. This can aid in structuring the database effectively and spotting potential issues before they arise during development.

• Documentation

An ER diagram can serve as a valuable piece of documentation, providing a clear and concise visual representation of the database's structure. This can be helpful for both the development team and any stakeholders, as it allows everyone to understand the system more easily.

• Analyzing Existing Systems

If you're working with an existing database system, creating an ER diagram can help you better understand its structure and operations. This can be particularly useful when making modifications or troubleshooting problems.

• Communication Tool

When communicating complex systems to stakeholders, ER diagrams can act as a simplified map that outlines how everything fits together without requiring technical knowledge.

• Educational Purposes

ER diagrams are commonly used in academic settings for teaching database design and related concepts. They provide students with a hands-on approach to understand how different elements of a database interact with each other.

In summary, ER diagrams are most commonly used during the design phase of a database project, but their versatility makes them an invaluable tool in various situations related to database understanding, analysis, communication, and education.

How to Database Modeling with ER Diagram?

Entity Relationship diagrams are graphical representations that model data in a database. They show entities (like users, products, orders), their attributes (like names, addresses, emails), and the relationships between these entities (like 'orders' placed by 'users'). Whether you're building a simple blog or a complex e-commerce site, ER diagrams help structure your data effectively. Here's a complete guide on database modeling with ER diagrams.

1. Understanding the Components of an ER Diagram

The building blocks of an ER diagram are entities, attributes, and relationships:

1.1 Entities

Entities represent objects or concepts in the database, like 'User' or 'Product.' They are typically nouns and are represented as rectangles in ER diagrams.

1.2 Attributes

Attributes are characteristics or properties of entities. For instance, a 'User' entity might have attributes like 'Name,' 'Email,' and 'Address.' Attributes are depicted as ovals connected to their respective entities.

1.3 Relationships

Relationships denote how entities interact with each other. For example, a 'User' might 'Place' an 'Order.' Relationships are represented as diamond shapes connecting related entities.

2. Developing an ER Diagram

Once you understand the components of ER diagrams, follow these steps to develop one:

2.1 Identify Entities

Determine all entities in your system. An e-commerce system, for example, might include 'User,' 'Product,' 'Order,' and 'Payment' entities.

2.2 Identify Attributes

Next, pinpoint all the attributes associated with each entity. Remember, these attributes should be atomic, meaning they can't be further divided into meaningful components.

2.3 Determine Relationships

Identify the interactions between your entities and how they relate to each other.

2.4 Review and Refine

After sketching your initial diagram, review and refine it for any errors or improvements. It's critical to ensure that the diagram accurately reflects your system's data requirements.

3. Leveraging ER Diagrams for Effective Database Design

ER diagrams provide a blueprint for designing databases. By visualizing the structure and relationships of data, they facilitate effective database design. Furthermore, they encourage normalization, which avoids redundancy and inconsistencies in data storage.

4. Using ER Diagram Tools

Many online tools, such as Boardmix, can assist you in creating detailed ER diagrams. They offer easy-to-use interfaces with drag-and-drop capabilities and provide options to export your diagrams in various formats.

To sum up, ER diagrams play a crucial role in database modeling. They provide a clear visual representation of the data and its interrelationships, paving the way for a well-structured and efficient database system. By understanding their components and developing them effectively, you can create a robust foundation for your database.

How to Create an ER Diagram on Boardmix?

There are some free ER templates on Boardmix and you can also create your own online. Creating an Entity-Relationship (ER) Diagram on Boardmix can be accomplished by following these steps:

• Log into Boardmix

The first step is to sign in to your Boardmix account. If you don't have one, you can create an account on the Boardmix website.

• Open a New Project

Once you are logged in, click on "New Project" or select a pre-existing project where you wish to add the ER Diagram.

• Choose ER Diagram Type

Boardmix usually offers several diagram types to choose from. Search for ER diagram and choose an Entity-Relationship Diagram template.

• Add Entities

Use the toolbar on the left side of the interface to add entities to your diagram. Simply click on the entity icon (typically a rectangle), then click on the diagram space to place it. You can label the entity as needed (e.g., "User", "Product", "Order").

• Add Attributes

Next, add attributes to your entities by clicking on the attribute icon (usually an oval) and then clicking on the diagram near the respective entity. Label these appropriately (e.g., "Name", "Email" for a "User" entity).

• Create Relationships

Establish relationships by clicking on the relationship icon (often a diamond) and then drawing lines between the entities you want to link.

• Review and Edit

Take a moment to review your diagram. Boardmix allows you to easily edit labels, move components around, and delete anything as needed.

• Save and Share

After you've verified that all information is correct, save your diagram. Boardmix also allows you to share your diagrams with others, export them into different formats, or print them directly.

In these easy steps, you can create an ER diagram using Boardmix that not only helps in effective database modeling but also improves communication within your team.

Join Boardmix to collaborate with your team.

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Sudip is a TOGAF Certified Solutions Architect with more than 15 years spent in the IT industry. He is now a full-time freelance technical writer and has contributed to several publications on various topics including the cloud, DevOps, SaaS, and cybersecurity. When not reading or writing, he is found playing a game of chess or in a squash court.

• Entity-Relationship Diagram
• Vertabelo Features

Get to know what an ER Diagram is through use cases and examples. Explore the notations for the essential components of an ERD.

Entity-relationship diagrams (ERDs) are used to visualize data and relationships among the entities of a system. At its core, an ER Diagram explains the logical structure and the flow of information within a data model. To do so, an entity-relationship diagram highlights entities, attributes , and relationships through predefined symbols for effective visualization of the architecture. Preparing an ERD is an essential first step towards designing a database, helping to analyze data flow and helping with due diligence of related system requirements.

When you Need an ER Diagram

An ER diagram is used to design database data models. If you are designing a new data model, preparing an ER Diagram helps with essential due diligence to capture and document system requirements. While in production, an ER diagram acts as a reference to improve database throughput , debug errors , and redesign architecture .

In addition to technical references, as ERDs are conceptual blueprint of the data flow within a system, they are also used to streamline business processes by identifying redundancies and bottlenecks within business entities.

In this article, we will go through a few ER diagram examples and how you can use Vertabelo to create ER diagrams to design efficient data models.

Objects in ER Diagrams

While there are several notations, the crow’s foot notation (also known as the information engineering notation or the IE notation) is prominently used in ER modeling due to its intuitive format and symbols. Let us delve into the representation of entities and relationships in crow’s foot notation to help model an ER diagram.

Entities represent data components within a database that could be living or non-living, real or abstract, so long as their data is stored in the database. Examples of entities include people, places, objects, events, or concepts. In ER diagrams, entities are usually depicted by rectangles, with the entity name at the top.

Client, City and Country Entities of a Data Model

An attribute is a description of the properties of an entity or relationship. For instance, the entity shipment could have attributes such as shipment ID ( id ), client_id , shipment time_created , billing_address , etc. In the crow’s foot notation, an attribute is represented as a line of text in the lower compartment of the corresponding entity’s rectangle.

A Snapshot Showing Various Attributes of the shipment Entity

Relationship

A relationship denotes how various entities interact with each other. For instance, in a database for a vehicle hiring service, a customer will form a relationship with the entity vehicle through the act of selecting . “Select” is, therefore, a relationship between the two entities.

The relationship between data in one table and data in another table is called cardinality . Specifically, the cardinality indicates the number of times one entity in a table can relate with the instances of another entity.

When drawing relationships in crow’s foot notation, there are two indicators: one for multiplicity (cardinality) while the other indicates whether a relationship is optional or mandatory. A mandatory relationship is shown by a line perpendicular to the relationship line, while an optional relationship is shown by an empty circle.

When drawing relationships in crow’s foot notation, there are two indicators: one for multiplicity (cardinality) while the other indicates whether a relationship is optional or mandatory. A mandatory relationship is shown by a line perpendicular to the relationship line, while an optional relationship is shown by an empty circle.

In data modeling, there are three types of cardinalities.

In a one-to-one association, one entity in a table can relate only once to an entity in another table. For instance, a customer can hire only one vehicle at a time, and a vehicle can be assigned to only one customer at a time.

In an entity-relationship diagram, one row in one table would correlate to only one row in another table. In the IE notation, a mandatory one-to-one relationship is represented by two lines perpendicular to the relationship line near both entity rectangles.

One-to-One Association With Mandatory Relationship

One-to-One Association With Optional Relationship

One-to-Many

A one-to-many relationship occurs when one instance of an entity can interact with multiple instances of another entity. For instance, one customer can hire several vehicles, while each vehicle can only be assigned to one customer at a time.

In the IE notation, a one-to-many relationship is represented by a single line at the end of the single instance, and a three-pronged crow’s-foot symbol facing the multiple instance rectangle.

One-to-Many Association With Mandatory Relationship

Many-to-Many

This type of cardinality occurs when multiple instances of one entity can relate with more than one instance of another set of entities. Imagine a scenario in which a customer could select multiple vehicles, and a vehicle could be leased by multiple customers within a certain time period. In the IE notation, a many-to-many relationship is depicted by crow’s feet at both ends of the relationship line.

Many-to-Many Association With Mandatory Relationship

Many-to-Many Association With Optional Relationship

While the above notations are in crow’s foot notation, there are a number of other ERD notation formats, the details of which can be found here .

Types of Data Models

There are three types of data models in ER modeling.

Conceptual and Logical Data Models 

The conceptual and logical data models describe how information is organized in a system regardless of the database used. The logical model specifies entities, attributes, and relationships between entities. It abstracts away from actual DBMS used in the implementation of the system.

Physical Data Model

The physical data model represents how the model will be built in the database. It shows the full table definition, including column names, column data types, table constraints, the primary key, the foreign keys, and the relationships between tables. The physical data model will be different for different RDBMS. For example, the data type for a given column may be different between MySQL and SQL Server.

This section appears incomplete. There is no explanation of what a conceptual model is (as opposed to logical). Also, the last sentence is incomplete. here .-->

How to Draw an ER Diagram

Let’s create an ER diagram for a data model using Vertabelo . Consider a simple scenario in which a client orders food at a restaurant. The client must purchase one or more menu items, while each menu item is served by one attendant.

1. First, identify the entities in your database. In this case, we have three entities: Client , Menu_Item , and Attendant .

2. The second step involves identifying the relationships between the selected entities.

• The Client is served a Menu_Item .
• The Attendant delivers the Menu_Item .

3. The third step involves identifying cardinalities .

• A Client can be assigned multiple Menu_Item s.
• An Attendant can only deliver one Menu_Item at a time before taking another order from the same Client (assumption) .

4. The fourth step is identifying entity attributes . Make sure that every attribute is mapped to only one entity; assign modifiers for those that belong to more than one.

Specify the primary key for each entity by using the most uniquely identifying attributes.

Setting the Attributes of an Entity Using the Vertabelo Property Editor

5. Once you have identified the entities, relationships, cardinalities, and attributes, you can now create your ER diagram. Here’s what our sample project will look like when designed using the crow’s foot (IE) notation.

Disclaimer: All the above diagrams were created using Vertabelo .

Data modeling challenges in the real world are often complex. Creating a precise ER diagram requires not only the right knowledge but also the right tools. Whether you are learning or designing a data model, Vertabelo acts as a great resource to your ERD designing needs. To find out more about Vertabelo features and how it can help, visit this article .

ER Diagram Example Use Cases

Let’s look at a few examples of the entity-relationship diagram to understand its use cases better.

Use Case: Data Model for a Freelancers Collective

You can find a detailed discussion of this ER diagram in A Data Model for a Freelancers Collective .

This model shows the logical structure that can be used in large freelance projects that require collaboration between distributed teams. Since the freelance world connects professionals in a wide range of fields with varying skill sets, you will need a tool to orchestrate the interactions among freelancers, teams, and clients. Let’s consider the Freelancer Collective platform as a project management solution that helps connect various actors to collaborate on a number of projects.

Our model is broken into four main subject areas: Freelancers , Customers & Projects , Teams , and Project Phases .

The Freelancers section includes all the information provided by freelancers when they register to use the application. The information relevant to our database includes the freelancer’s skills, skill level, and skill sets. We also need to understand the freelancer’s availability to be sure that they can deliver an assigned project.

The second focus area is Customers & Projects . Customers use the system to upload their projects and connect with freelancers. This section includes the customer , project , project_status_history , skill , and project_outcome tables. Customers have also registered to use the application, and the system extracts their details from a special form. Project details are revealed when the client posts a project.

The third section is for the teams . This subject area forms a team and assigns them a project based on the freelancer’s ability and skill sets. The team is the major entity for this section. Other tables in this section include team_member , in_charge , freelancer , phase_plan , and project .

The final section involves the Project phases . Some entities in this section have appeared in other subject areas, and these include freelancer , customer , project_outcome , and project . The new entities in this set include phase_history , phase_catalog , and phase_plan .

Relationships

When a client posts a project, the system checks for the skill requirements and the availability of freelancers. The system then creates a team of freelancers who will collaborate on the project, which is now divided into phases. This system works as a supervisor, looking for the best collective that will work on a project, and assigning different project phases to different freelancers depending on the availability and skill level.

Collectively, this is how the data model can look:

Use Case: Peer-to-Peer Lending Data Model

You can find a detailed discussion of this ER diagram in A Peer-to-Peer Lending Platform Data Model .

Peer-to-peer lending refers to the practice of multiple lenders and borrowers transacting through a common platform. A peer-to-peer lending platform here essentially acts as a liaison between the parties to enable the process of lending, while ensuring necessary compliance and maintaining data security of all parties.

Let’s understand the data model by considering a scenario.

The entities are the following:

• A borrower requests a loan of a specific amount through the peer-to-peer lending platform after submitting details of his credit score, default history, tax returns, etc. Additional details of the loan may include repayment duration and preferred interest rate window.
• The lender(s) register(s) on the platform by entering its respective financial details, including KYC, tax returns, etc. It also enters its lending preferences, including the loan duration, interest rate, amount, borrower credit score, etc.

The borrower asks for funding from an investor, who releases the funds through the loan fulfilment and repayment subsystem. This subsystem forms an associative relationship between the two entities.

The lending platform here essentially acts as the interface, connecting would-be investors with potential borrowers. This reduces the complexity of access to financing by allowing users to determine their own lending rates. The data model that can support this platform is outlined below:

An ER Diagram Is a Must

An ER diagram provides an easy way to represent your data model at a higher level of abstraction. With marked-up relationships among data sets, an ER diagram is perfect for process modeling and system debugging. In this article, we discussed the core concepts of ER modeling, with practical examples using Vertabelo for pragmatic experience. Whether you are learning or working on real-time data modeling, creating an ER diagram is a must.

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What is an Entity Relationship Diagram (ERD)?

What are your erd needs, i want to make my own erd in lucidchart., i want to make an erd from a lucidchart template..

Learn the essentials of ER Diagrams and ER Models, along with their origins, uses, examples, components, limitations and guidelines on how to draw them using our ER diagram tool.

10 minute read

Want to make an ERD of your own? Try Lucidchart. It's quick, easy, and completely free.

What is an ER diagram?

An Entity Relationship (ER) Diagram is a type of flowchart that illustrates how “entities” such as people, objects or concepts relate to each other within a system. ER Diagrams are most often used to design or debug relational databases in the fields of software engineering, business information systems, education and research. Also known as ERDs or ER Models, they use a defined set of symbols such as rectangles, diamonds, ovals and connecting lines to depict the interconnectedness of entities, relationships and their attributes. They mirror grammatical structure, with entities as nouns and relationships as verbs.

ER diagrams are related to data structure diagrams (DSDs), which focus on the relationships of elements within entities instead of relationships between entities themselves. ER diagrams also are often used in conjunction with data flow diagrams (DFDs), which map out the flow of information for processes or systems.

History of ER models

Peter Chen (a.k.a. Peter Pin-Shan Chen), currently a faculty member at Carnegie-Mellon University in Pittsburgh, is credited with developing ER modeling for database design in the 1970s. While serving as an assistant professor at MIT’s Sloan School of Management, he published a seminal paper in 1976 titled “The Entity-Relationship Model: Toward a Unified View of Data.”

In a broader sense, the depiction of the interconnectedness of things dates back to least ancient Greece, with the works of Aristotle, Socrates and Plato. It’s seen more recently in the 19th and 20th Century  works of philosopher-logicians like Charles Sanders Peirce and Gottlob Frege.

By the 1960s and 1970s, Charles Bachman (above) and A.P.G. Brown were working with close predecessors of Chen’s approach.  Bachman developed a type of Data Structure Diagram, named after him as the Bachman Diagram. Brown published works on real-world systems modeling.  James Martin added ERD refinements. The work of Chen, Bachman, Brown, Martin and others also contributed to the development of Unified Modeling Language (UML), widely used in software design.         

Uses of entity relationship diagrams

Database design:, database troubleshooting:, business information systems:, business process re-engineering (bpr):, the components and features of an er diagram.

ER Diagrams are composed of entities, relationships and attributes. They also depict cardinality, which defines relationships in terms of numbers. Here’s a glossary:

A definable thing—such as a person, object, concept or event—that can have data stored about it. Think of entities as nouns. Examples: a customer, student, car or product. Typically shown as a rectangle.

Entity type:

Entity set:, entity categories:, strong entity, weak entity, entity keys:, candidate key:, primary key:, foreign key:, relationship.

How entities act upon each other or are associated with each other. Think of relationships as verbs. For example, the named student might register for a course. The two entities would be the student and the course, and the relationship depicted is the act of enrolling, connecting the two entities in that way. Relationships are typically shown as diamonds or labels directly on the connecting lines.

Recursive relationship:

A property or characteristic of an entity. Often shown as an oval or circle.

Descriptive attribute:

Attribute categories:, multi-value:, single-value:, one-to-one example, one-to-many example (or many-to-one, depending on the relationship direction):, many-to-many example:, cardinality views:, cardinality constraints:, mapping natural language.

ER components can be equated to parts of speech, as Peter Chen did. This shows how an ER Diagram compares to a grammar diagram:

Common noun:

Proper noun:.

The database query language ERROL actually mimics natural language constructs. ERROL is based on reshaped relational algebra (RRA) and works with ER models, capturing their linguistic aspects.

ERD symbols and notations

There are several notation systems, which are similar but vary in a few specifics.

Chen notation style

Crow’s foot/martin/information engineering style, bachman style, idef1x style, barker style.

Following are examples of ERD diagrams made in each system.

Conceptual, logical and physical data models

ER models and data models are typically drawn at up to three levels of detail:

Conceptual data model:

Logical data model:, physical data model:.

Note that similar detail and scope levels exist in other types of diagrams, such as data flow diagrams, but that it contrasts with software engineering’s three schema approach, which divides the information a bit differently. Sometimes, engineers will branch out ER diagrams with additional hierarchies to add necessary information levels for database design. For example, they may add groupings by extend up with superclasses and down with subclasses.

Limitations of ER diagrams and models

Only for relational data:, not for unstructured data:, difficulty integrating with an existing database:, how to draw a basic er diagram, purpose and scope:, relationships:, attributes:, cardinality:, more tips for er diagrams.

Show the level of detail necessary for your purpose. You might want to draw a conceptual, logical or physical model, depending on the detail needed. (See above for descriptions of those levels.)

Watch for redundant entities or relationships.

If you’re troubleshooting a database problem, watch for holes in relationships or missing entities or attributes.

Make sure all your entities and relationships are labeled.

You can translate relational tables and ER diagrams back and forth, if that helps you achieve your goal.

Make sure the ER diagram supports all the data you need to store.

• There may be different valid approaches to an ER diagram. As long as it provides the necessary information for its scope and purpose, it’s good.

Helpful Resources

• How to Draw an ER Diagram
• Entity-Relationship Diagram Symbols and Notation

Lucidchart lets you easily create ER diagrams, with smooth placement of shapes, lines and labels. With all editing taking place in the cloud, it’s easy to collaborate with colleagues. You can share your diagram digitally or via print.