Entity Relationship Diagram

Use Mermaid erDiagram to model tables, entities, fields, keys, and cardinality.

Basic template

erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string CustomerID PK
        string Name
    }

Core syntax

• Start with erDiagram
• Define relationships like A ||--o{ B : label
• Common cardinality markers:
  • || exactly one
  • o| zero or one
  • |{ one or many
  • o{ zero or many
• Add fields inside entity blocks
• Mark key columns with labels like PK and FK

Common example

erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ ORDER_LINE : contains
    PRODUCT ||--o{ ORDER_LINE : appears_on

    CUSTOMER {
        string CustomerID PK
        string Name
        string Email
    }

    ORDER {
        string OrderID PK
        date OrderDate
        string CustomerID FK
    }

    ORDER_LINE {
        string OrderLineID PK
        string OrderID FK
        string ProductID FK
        int Quantity
    }

    PRODUCT {
        string ProductID PK
        string Name
        decimal Price
    }

Good for

• Database design
• Data integration mapping
• System documentation

Watch for

• Keep naming consistent with the actual schema
• Mermaid ER diagrams are ideal for conceptual and logical models, not every physical storage detail