Mermaid ER Diagram Maker: Entity Relationship Diagrams

Mermaid ER Diagram Syntax

Entity relationship diagrams map database schemas visually. Each entity is a table, each attribute is a column, and the lines between entities show how tables relate through foreign keys. Mermaid’s ER syntax lets you define all of this in plain text and see the rendered diagram update as you type.

Entities and Attributes

Define an entity with its attributes inside curly braces:

erDiagram
    USER {
        int id PK
        string email UK
        string name
        string password_hash
        timestamp created_at
    }

Each attribute line has three parts: type name constraint. The constraint (PK, FK, UK) is optional.

Relationships

Relationships connect entities and specify cardinality:

CUSTOMER ||--o{ ORDER : "places"

This reads as: one CUSTOMER places zero or more ORDERs. The label after the colon describes the relationship verb.

Cardinality Symbols

The relationship line is -- between the two sides. So ||--o{ means “exactly one on the left, zero or more on the right.”

Common Patterns

Modeling Patterns

One to many (the most common)

erDiagram
    AUTHOR ||--o{ BOOK : writes
    AUTHOR {
        int id PK
        string name
    }
    BOOK {
        int id PK
        int author_id FK
        string title
        date published
    }

Self-referencing

erDiagram
    EMPLOYEE ||--o{ EMPLOYEE : "manages"
    EMPLOYEE {
        int id PK
        int manager_id FK
        string name
        string role
    }

Many to many through junction

erDiagram
    STUDENT }o--|| ENROLLMENT : enrolls
    ENROLLMENT ||--o{ COURSE : "enrolled in"
    STUDENT {
        int id PK
        string name
    }
    ENROLLMENT {
        int id PK
        int student_id FK
        int course_id FK
        date enrolled_at
        string grade
    }
    COURSE {
        int id PK
        string title
        int credits
    }

Always model many-to-many as two one-to-many relationships through an explicit junction table. This matches the actual database schema and lets you add attributes to the junction (enrolled_at, grade, etc.).

Tips for Readable ER Diagrams

Keep entity names uppercase and singular (USER not users). Use short, descriptive relationship labels. Put the “parent” entity on the left side of the relationship (the one with ||). For large schemas, break the diagram into domain-specific sections rather than cramming everything into one diagram.

For documenting API interactions with these entities, try the Mermaid Sequence Diagram. For process flows that operate on this data, use the Mermaid Flowchart.