Class Diagram vs ERD: A Complete Side-by-Side Comparison

Updated on: 27 October 2025 | 10 min read

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Class Diagram vs ERD: A Complete Side-by-Side Comparison

If you’ve ever confused an ERD with a class diagram, you’re not alone. ERDs show how data is structured in a database, while class diagrams show how that data behaves in software. Use ERDs for database design and class diagrams for object-oriented planning. This guide breaks down their differences, notations, mapping patterns, conversion steps, real-world examples, a migration checklist, and templates to build your own models quickly.

ER Diagram vs Class Diagram: Key Differences

At first glance, ERDs and class diagrams might look similar—boxes, lines, and labels—but they serve very different purposes. ERDs focus on how data is structured inside a database, while class diagrams focus on how objects behave and interact inside software. In short: ERDs are about data; class diagrams are about design and behavior. Here are the key differences between Class and ER diagrams.

Aspect	ERD (Entity-Relationship Diagram)	UML Class Diagram
Purpose & lifecycle stage	Database design, normalization, migration planning	Application design, OO modeling, code architecture
Core focus	Entities, attributes, and relationships	Classes, attributes, methods/operations, and object behavior
Notation	Attributes only; relationships labeled; cardinality shows “how many”	Attributes + methods; associations labeled; multiplicity shows “how many instances”
Level of abstraction	Conceptual or logical view of data	Implementation/system view of objects
Who uses it	Database designers, DBAs, data architects	Software developers, architects, system designers
Inheritance/Generalization	Rarely shown; not standard	Supported; shows class hierarchies and interfaces
Behavior/Methods	Not included	Central; methods show operations classes can perform
Strengths	Clear visualization of relationships and participation, good for normalization	Good for modeling object behavior, system structure, and cardinality of associations
Weaknesses	Cannot show methods or behavior; limited for software design	Can be more complex; not ideal for database normalization or direct schema planning
Example	Customer—Order relationship, with cardinality	Customer class with attributes (name, ID) and methods (placeOrder(), cancelOrder())

What Is an ERD (Entity-Relationship Diagram)?

An Entity-Relationship Diagram, or ERD, is a visual way to show how data is organized and connected inside a system. Think of it as a map of your database—it defines what information you’re storing, how it’s grouped, and how different parts relate to each other.

Entity-Relationship Diagram Example

Database designers and DBAs create ERDs to plan and refine data structures before writing a single line of SQL. They’re also helpful for normalizing data, preventing redundancy, and simplifying migrations or database updates later on.

What Is a UML Class Diagram?

A UML class diagram shows the static structure of an object-oriented system. It visualizes how different parts of your software—classes, their properties, and how they interact—fit together. Think of it as a blueprint for your codebase before you start building it.

Class Diagram Example

Software architects and developers create class diagrams to plan system structure, design APIs, or prepare for code generation. They’re also handy for documenting large systems or communicating architecture across teams before coding begins.

How to Convert an ERD to a Class Diagram

Converting an ERD to a class diagram isn’t just about drawing boxes. It’s about turning database structures into software objects with behavior. Here’s a step-by-step breakdown:

Step 1: Identify all entities and turn them into classes

Look at each entity in your ERD. These are typically nouns like Customer, Order, or Product.
Action: Create a class for each entity. Name the class exactly like the entity (capitalize it if you’re following standard coding conventions).
Tip: Keep the name simple and singular (Customer, not Customers).

Step 2: Map attributes to class properties

Each attribute in your ERD becomes a property (or field) in the class.
Action: For each entity, list all its attributes as class properties.
Example: Customer entity has customer_id, name, email. In the class diagram, the Customer class will have these as attributes.
Tip: If your database uses nullable fields, note whether these properties are optional in your class.

Step 3: Convert relationships to associations

Look at how entities are connected in your ERD. Each relationship becomes an association in the class diagram.
One-to-one: Draw a direct association line between classes.
One-to-many: Draw a line with multiplicity 1..* on the “many” side.
Many-to-many: Create a new associative class to represent the relationship. This class will often have attributes of its own.
Action: For each relationship, decide whether it becomes an association or an associative class. Add multiplicities.

Step 4: Handle composite keys

If an entity uses a composite key (two or more attributes together form a primary key), this often becomes a value object or a combination of properties in the class.
Action: Identify composite keys and decide whether to create a new class/value object or just include them as multiple attributes.
Example: An Enrollment entity uses StudentID + CourseID → Class Enrollment has both as attributes.

Step 5: Handle weak entities

Weak entities depend on a parent entity for their identity.
Action: In your class diagram, nest the weak entity as a composed object inside the parent class.
Example: OrderItem is a weak entity dependent on Order. In the class diagram, the Order class contains a list of OrderItem objects.

Step 6: Add multiplicity and cardinality

Multiplicity tells you how many instances of one class relate to another.
Action: Translate the ERD cardinality into class diagram multiplicity notation.
Example: One Customer can have many Orders → Customer class has 0..* association to Order.

Step 7: Add methods and behavior

ERDs don’t include methods, but class diagrams do. Think about what operations each class should perform.
Action: Add relevant methods that represent actions in the system.
Example: Customer class → place_order(), cancel_order(). Order class → calculate_total().

Step 8: Review and refine the design

Check the diagram for consistency. Make sure associations and multiplicities make sense, and that weak entities and composite keys are modeled properly.
Action: Denormalize if needed for performance or coding convenience, but keep relationships clear.
Tip: Ensure methods reflect real behavior and don’t clutter the diagram.

When ERD Is Better (Use Cases)

ERDs shine when your focus is on data first. They’re perfect for understanding, organizing, and optimizing how information is stored. Use an ERD in situations like:

Database-first projects: Planning your schema before you write any code.
Normalization: Reducing data redundancy and ensuring data integrity.
Data migration: Moving or consolidating data between systems.
Reporting / data warehouses: Designing structures for analytics and reporting.
Database governance: Keeping track of relationships, dependencies, and compliance requirements.

When Class Diagram Is Better (Use Cases)

Class diagrams are best when your focus is on how the software works—not just the data it stores. They’re ideal in scenarios like:

Object-oriented (OO) design: Modeling classes, objects, and their interactions before coding.
API design: Planning endpoints and the data each class exposes or consumes.
Domain-driven design (DDD) modeling: Capturing complex business logic and entities in your software.
Code generation: Preparing models that can directly translate into code or ORM classes.

Helpful Resources

Different Types of ERD

Explore the different types of ER diagrams, including conceptual, logical, and physical models. Learn how each type helps visualize database structure and relationships effectively.

ER Diagram Symbols

Learn about key ER diagram symbols and their meanings, including entities, relationships, attributes, and keys.

Class Diagram Symbols

Discover the essential class diagram symbols and their meanings, including classes, attributes, operations, and relationships.

Class Diagram Templates

Explore ready-to-use class diagram templates to design and visualize object-oriented systems.

How to Make a Class Diagram

Learn how to make a class diagram step by step, from identifying classes and attributes to defining relationships and methods.

Class Diagram Relationships

Understand class diagram relationships, including associations, aggregations, compositions, and inheritance.

Common Notation Variants & Enterprise Preferences

Not all diagrams look the same—different organizations and tools use slightly different notation styles. Here’s a quick overview:

ERD notations

Chen notation: Uses rectangles for entities, diamonds for relationships, and ovals for attributes. It’s classic and easy to read for conceptual models.
Crow’s Foot notation: Uses rectangles for entities and lines with “crow’s feet” to indicate cardinality (one-to-many, many-to-many). Very popular in enterprise database design.
UML-style ERD: Some ERDs adopt UML class diagram symbols for consistency when modeling both data and objects.

Class diagram variants

UML versions: UML has evolved over time (UML 1.x, UML 2.x), but the main class diagram symbols remain consistent: classes, attributes, methods, associations, and multiplicities.
Dialects: Some teams use simplified or customized versions for readability, like showing only attributes or ignoring visibility for small diagrams.

Tooling & enterprise preferences

Support for both diagrams: Tools that let you create ERDs and class diagrams in one environment help maintain consistency.
Creately: Supports both ERD and UML class diagrams with drag-and-drop shapes, real-time collaboration, built-in notation styles (Chen, Crow’s Foot, UML), customizable templates, and easy export options (PNG, SVG, PDF). This makes it easy to share diagrams with teams or include them in documentation.

Best Practices & Checklist

Follow these practical steps to get the most out of ERDs and class diagrams:

Start conceptually with ERD for data-critical systems: Begin with a high-level ERD to map entities, relationships, and rules before diving into software design. This ensures your data structure is solid from the start.
Keep class diagrams aligned with code: Update class diagrams whenever you refactor or add new functionality. This keeps your design documentation accurate and useful for current development.
Collaborative workflow: DBAs and developers should sync through canonical models. Use diagrams as a common language to avoid miscommunication and ensure database and application layers stay in harmony.
Versioning and migration: Keep track of diagram versions and link them to migration scripts or schema changes. This helps maintain a clear history and makes audits or rollbacks easier.
Use templates and notation consistently: Stick to one notation style per project and leverage templates to save time and maintain clarity across your diagrams.

Ready-to-Use Templates for ER Diagrams and Class Diagrams

Here are some pre-made ERD templates for you to get started with.

ATM ERD Diagram

ER Diagram for Online Shopping System

ER Diagram for Medical Records Management

ER Diagram for Tracking, Borrowing, and Returning Activities

And here are some Class diagram templates to get a head start.

ATM Class Diagram

Video Game Class Diagram

Enterprise Cloud Class Diagram

Bank Loan Class Diagram

FAQs About UML Class Diagram vs ER Diagram

Are there similarities between class diagrams and ER diagram?

Yes, both diagrams use boxes to represent entities/classes and lines to show relationships/associations. They both can include attributes and multiplicities/cardinality. The key difference is that ERDs focus on data storage, while class diagrams focus on objects and behavior in software.

How do I handle many-to-many relationships when converting an ERD to a class diagram?

Create an associative class in the class diagram to manage the relationship. This class can have its own attributes and methods if needed.

Are ERDs only useful for relational databases?

Mostly, but they can also be adapted for NoSQL or document databases to visualize entities, relationships, and key constraints, even if the implementation is different.

Can class diagrams show database-specific constraints?

Not directly. Class diagrams focus on objects and behavior. You can include multiplicities and associations, but database-specific rules like foreign keys or indexes are usually not shown.

Can ERDs and class diagrams overlap in content?

Yes. Attributes in an ERD often map to class attributes, and relationships may map to associations. However, class diagrams also include methods and behavior, which ERDs do not.

What are some common mistakes to avoid when working with ERDs and class diagrams?

A few pitfalls include mixing notations, overcomplicating relationships, ignoring updates when the system evolves, and forgetting to document assumptions. Another common issue is treating class diagrams as databases or vice versa — each has a specific purpose and should be kept consistent but distinct.

Author

Amanda Athuraliya Communications Specialist

Amanda Athuraliya is the communication specialist/content writer at Creately, online diagramming and collaboration tool. She is an avid reader, a budding writer and a passionate researcher who loves to write about all kinds of topics.

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