Mermaid ER Diagram Syntax: Entities, Attributes, Keys, and Crow's-Foot Cardinality

Example

erDiagram
    USERS {
        text id PK
        text email
        text plan
        int created_at
    }
    DIAGRAMS {
        text id PK
        text code
        text r2_key
        int view_count
        int last_viewed_at
        text user_id FK
    }
    USERS ||--o{ DIAGRAMS : "owns"

Syntax

erDiagram
    USER {
        uuid id PK
        text email UK
        timestamp created_at
    }
    POST {
        uuid id PK
        uuid user_id FK
        text title
        text body "markdown"
    }

erDiagram
    USER ||--o{ POST : writes
    POST ||--|{ COMMENT : has
    USER }o--o{ ROLE : assigned
    CART ||..|| ORDER : becomes

erDiagram
    "Order Line" {
        int qty
    }
    ORDER ||--|{ "Order Line" : contains

Common mistakes

These are the parse errors that come up most, including in code written by ChatGPT and Claude. Paste any failing diagram into the editor and it marks the offending line.

• Cardinality token reversed: POST o{--|| USER.

  Fix: The symbol closest to each entity describes that entity's side: USER ||--o{ POST means one user, zero-or-more posts.

• Attribute type and name swapped (id uuid PK).

  Fix: Type comes first: uuid id PK. Swapping them renders but labels the column "uuid".

• Lowercase entity names mixed with uppercase ones.

  Fix: Mermaid accepts both, but the convention is uppercase. Pick one style; AI tools follow whatever your example uses.

Real-world examples

Complete, copy-ready entity relationships for common situations. Each one opens in the editor with a click.

• E-commerce Database ER Diagram with Orders and Payments

  A Mermaid ER diagram for an online store: customers, addresses, orders, order items, products, categories, and payments, with typed keys and cardinality.

• Blog CMS Database ER Diagram: Posts, Tags, and Comments

  A Mermaid ER diagram for a blog or CMS: users, posts, comments, tags through a post_tags join table, and media attachments, with typed keys and cardinality.

FAQ