Entity Relationship Diagram

Entity Relationship (ER) diagrams document database structures and the relationships between tables. Mermaid Studio provides full language support for designing and maintaining your data models.

Basic Example

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : "ordered in"

    CUSTOMER {
        int id PK
        string name
        string email
    }
    ORDER {
        int id PK
        date created
        int customer_id FK
    }

What You Get

ER diagrams have comprehensive IDE support:

• Syntax highlighting - Distinct colors for entities, attributes, and relationships
• Code completion - Suggestions for relationship types and attribute modifiers
• Refactoring - Rename entities and update all references
• Find usages - See where entities are referenced in relationships
• Code Vision - Inline hints showing relationship counts
• Navigation bar - Quick access to entities in large schemas
• Code folding - Collapse entity definitions
• Formatting - Auto-format your diagram code
• Inspections - Detect undefined entities and invalid relationships

Quick Syntax Reference

Relationship cardinality:

• || - Exactly one
• o| - Zero or one
• }| - One or more
• o{ - Zero or more

Common patterns:

• ||--o{ - One to many (optional)
• ||--|{ - One to many (required)
• }o--o{ - Many to many

Entity attributes:

erDiagram
    USER {
        int id PK
        string email UK
        int role_id FK
        string name
    }

Attribute modifiers: PK (primary key), FK (foreign key), UK (unique key)

For the complete syntax reference, see the Mermaid.js ER Diagram documentation.