Free Online ER Diagram Editor

Example ER Diagram

erDiagram
  CUSTOMER ||--o{ ORDER : places
  ORDER ||--|{ LINE_ITEM : contains
  CUSTOMER {
    int id
    string name
  }
  ORDER {
    int id
    date createdAt
  }

Why Use This ER Diagram Editor?

How to Create an ER Diagram

1. Start with 'erDiagram' keyword to initialize your entity-relationship diagram
2. Define entities using simple names that represent database tables or objects
3. Add attributes to each entity using the format: entity { type attribute }
4. Specify attribute types like string, int, date, etc. to document data types
5. Define relationships between entities using relationship lines with cardinality
6. Use relationship syntax: EntityA ||--o{ EntityB : 'relationship name'
7. Set cardinality: || (exactly one), |o (zero or one), }o (zero or more), }| (one or more)
8. Label relationships with descriptive names to clarify the nature of connections

Best Practices for ER Diagrams

• Use clear entity names: Choose names that reflect real-world objects or database tables
• Document all attributes: Include key attributes that define each entity's characteristics
• Show primary keys: Mark or highlight attributes that serve as primary keys
• Specify cardinality correctly: Understand one-to-one, one-to-many, and many-to-many relationships
• Name relationships meaningfully: Use verb phrases that describe how entities relate
• Normalize your design: Avoid redundant data by properly structuring entities and relationships
• Consider scalability: Think about how relationships will perform with large datasets
• Keep entity count manageable: If you have more than 15-20 entities, consider multiple diagrams or subsystems

Advanced ER Diagram Features

Common ER Diagram Mistakes

• ⚠️Wrong cardinality symbols - || means exactly one, |o means zero or one, }o means zero or many, }| means one or many
• ⚠️Missing primary keys - every entity should have a primary key attribute identified
• ⚠️Redundant relationships - if A relates to C through B, you usually don't need a direct A-C relationship
• ⚠️Poorly named entities - use singular nouns (Customer, Order) not plural or actions
• ⚠️Many-to-many without junction table - most M:N relationships need a junction entity for implementation
• ⚠️Attributes in wrong entities - place attributes in the entity they describe, not related entities
• ⚠️Circular dependencies - review relationships to avoid problematic circular references
• ⚠️Mixing logical and physical models - decide if you're modeling concepts or actual database tables

When to Use ER Diagrams vs Other Diagrams