Mermaid ER Diagrams

ER (Entity-Relationship) diagrams model database structures.

Mermaid Entity-Relationship Diagrams (erDiagram) are specialized layouts designed explicitly for schema modeling and database design. They focus on Entities (database tables), Attributes (columns with data types), and Relationships (foreign key connections with structural cardinality).

Mermaid uses Crow's Foot-style relationship notation.

Basic ER Syntax

erDiagram
    CUSTOMER {
        string name
        int age
    }

    ORDER {
        int orderNumber
        date orderDate
    }

    CUSTOMER ||--o{ ORDER : places

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1. Declaring Entities and Attributes

An entity block represents a database table. Inside the curly braces {}, you define the fields using the format: DataType AttributeName Keys "Comments".

erDiagram
    CUSTOMER {
        int customer_id PK
        string first_name
        string email UK "Primary contact"
    }

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Cardinality

Relationships show how rows in one table connect to rows in another. The syntax uses a combination of bars (|) and circles (o) or crow's feet ({ or }) to define constraints on both sides of the line:

Examples of Complete Connectors

• A ||--|| B : Exactly one A connects to exactly one B.
• A ||--o{ B : One A connects to zero or many B rows (Most common Parent-Child setup).
• A }|--|{ B : Many-to-Many connection (Typically requires a join table in actual implementation).

Keys

You can flag special keys by appending these shortcuts after the attribute name:

• PK : Primary Key
• FK : Foreign Key
• UK : Unique Key

Attributes can identify keys.

erDiagram
    USER ||--o{ ORDER : places

    USER {
        int userId PK
        string name
    }

    ORDER {
        int orderId PK
        int amount
    }

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PK marks a primary key.

The source also describes FK for foreign keys and ? for nullable attributes.

Blog Database Example

erDiagram
    USER ||--o{ POST : writes

    USER {
        int id PK
        string username
    }

    POST {
        int id PK
        string title
    }

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Comprehensive E-Commerce Example

Here is a multi-table database schema displaying structural constraints, key assignments, data types, and text-labeled relationship directions:

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    PRODUCT ||--o{ LINE-ITEM : ordered-in

    CUSTOMER {
        int id PK
        string name
        string email UK
        string status
    }

    ORDER {
        int id PK
        int customer_id FK
        date order_date
        string status
    }

    LINE-ITEM {
        int id PK
        int order_id FK
        int product_id FK
        int quantity
        price unit_price
    }

    PRODUCT {
        int id PK
        string sku UK
        string title
        int stock_level
    }

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Layout and Advanced Tips

• Handling Hyphens: If your table or entity name contains a hyphen, wrap it in double quotes when assigning relationships (e.g., "LINE-ITEM" ||--|| ORDER). Inside the curly braces block definition, you can use the name directly without quotes.
• Relationship Labels: Always append a trailing description string (e.g., : "places") at the end of relationship lines to clearly define the action between entities.
• No Code Injection: Attributes do not support complex constraint language (like NOT NULL or DEFAULT). Keep those details tucked inside the "comment string" at the end of the field row.