ER Diagram Template
Design your database schema visually. Define tables, fields, and relationships between entities before writing a single line of SQL.
Use this templateWhat you get
- Table blocks with field type annotations
- PK / FK labels for quick reference
- Relationship connectors between tables
What this template is for
This ER diagram template gives you a starting point for mapping the entities, attributes, and relationships in any relational database. Use it to design a schema before you write migrations, communicate a data model to stakeholders, or document an existing database for a new team member. An entity-relationship diagram makes foreign keys and join conditions visible at a glance — which is far faster than reading a schema dump or asking the person who built it.
When to use this template
- Design a new application's database schema before writing the first migration.
- Document an existing production database so new engineers can understand the data model.
- Communicate a proposed schema change to a technical lead or architect for review.
- Map the relationships between tables when debugging a slow query or missing join.
- Plan a multi-tenant SaaS schema with tenant isolation boundaries drawn explicitly.
- Model an e-commerce database with products, orders, customers, and inventory tables.
How to use it
- 1Identify the main entities — the things your system needs to store, such as User, Order, or Product.
- 2List the key attributes of each entity and mark the primary key.
- 3Draw a line between entities that are related; label the relationship verb (places, contains, belongs to).
- 4Add cardinality notation to each relationship: one-to-one, one-to-many, or many-to-many.
- 5For many-to-many relationships, add a junction table with its own primary key and foreign keys.
Quick example
E-commerce ER diagram
How it compares to similar tools
ER diagram vs. class diagram (UML)
An ER diagram models data — entities, their attributes, and relationships — and maps cleanly to relational database tables. A UML class diagram models software — classes, methods, inheritance, and behavior. Use an ERD when designing or documenting a database schema; use a class diagram when designing object-oriented code. The same domain concept appears in both, but the focus is different: data structure vs. code structure.
ER diagram vs. data flow diagram
An ERD is static — it shows what data exists and how it relates. A data flow diagram is dynamic — it shows how data moves between processes and storage over time. Use an ERD when the question is 'what does our data look like'; use a DFD when the question is 'how does data move through the system'.
Chen notation vs. Crow's foot notation
Chen notation uses diamonds for relationships and explicit cardinality labels (1, N, M); it is more academic and verbose. Crow's foot uses small forked symbols at the line ends to show cardinality (one, many, optional); it is the de facto standard in modern database tools because it is more compact. Pick one notation and stay consistent — mixing the two in one diagram is the fastest way to confuse readers.
Common mistakes to avoid
Missing or wrong cardinality
A line between User and Order without cardinality markers does not say whether one user can have many orders or just one. Every relationship line must show cardinality on both ends. Getting this wrong upstream causes schema errors that are expensive to fix after data exists.
Skipping junction tables for many-to-many
A direct many-to-many line between Student and Course cannot be implemented in a relational database without a junction table (e.g. Enrollment). Always model the junction entity explicitly in the ERD — it is also usually where useful attributes like enrollment_date live.
Attributes that are really entities
Putting 'addresses' as a comma-separated attribute on User works for a sketch but fails at scale. If an attribute can have multiple values, change over time, or carry its own attributes (street, city, country), promote it to its own entity with a relationship back to User.
Ignoring primary keys
Every entity needs a primary key (usually 'id'). Missing primary keys are not a notation issue — they are a sign that the model has not been thought through. Mark them explicitly so anyone reading the diagram knows uniqueness rules.
One giant diagram for the whole database
Past 20-30 entities the diagram becomes unreadable. Split by bounded context or feature area: a User & Auth diagram, an Orders & Payments diagram, etc. Cross-context relationships can be shown by referencing entity names without redrawing all attributes.
Frequently asked questions
What is an ER diagram?+
An entity-relationship (ER) diagram is a visual model of the data in a system. It shows entities (things you store data about, like User or Order), the attributes of each entity, and the relationships between entities (one-to-one, one-to-many, many-to-many). It is the standard tool for designing or documenting relational databases.
What are the three main components of an ER diagram?+
Entities (rectangles — the things you store), attributes (ovals or fields inside the entity — properties of those things), and relationships (lines or diamonds connecting entities — how things relate). Cardinality markers on the relationship lines specify how many of each entity can be related.
What does cardinality mean in an ER diagram?+
Cardinality describes how many instances of one entity can be related to instances of another. The common cases are one-to-one (one User has one Profile), one-to-many (one User has many Orders), and many-to-many (Students take Courses, and Courses have many Students). Each notation has its own way of marking these on the line.
How do I convert an ER diagram to SQL tables?+
Each entity becomes a table, each attribute becomes a column, and the primary key column gets a NOT NULL UNIQUE constraint. One-to-many relationships are implemented by putting a foreign key column on the 'many' side. Many-to-many relationships require a junction table with foreign keys to both sides. Many tools can auto-generate SQL DDL from an ERD.
What is the difference between conceptual, logical, and physical ER diagrams?+
Conceptual is high-level, used in early design — just entities and relationships, no attributes or keys. Logical adds attributes, primary and foreign keys, and is database-agnostic. Physical includes specific data types, indexes, and constraints for a particular database engine. Most modern projects skip straight from conceptual to physical, but keeping the three levels separate is helpful for large systems.
Can I draw an ER diagram online for free?+
Yes. Open the CodePic ER diagram template and drag entities onto the canvas, add attributes inside each box, draw relationships with Crow's foot cardinality. Export to PNG, SVG, or share a live link. No sign-up required.
Start editing online
Open the template in CodePic, replace the sample nodes, and turn it into your own study board in a few minutes.
See examples: /templates/erd/examples
