Entity Relationship Diagram

Entity Relationship (ER) diagrams are used to model the relationships between entities in a database system. They show the logical structure of databases.

Syntax

Basic Elements

• Entities: entity EntityName
• Attributes: Listed within entities
• Relationships: Various types of connections between entities
• Cardinality: |o--o|, }o--o{, etc.

Relationship Types

• One-to-One: ||--||
• One-to-Many: ||--o{
• Many-to-One: }o--||
• Many-to-Many: }o--o{

Basic Example

Code:

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

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Advanced Example

Here's a more complex ER diagram showing a blog system:

Code:

erDiagram
    USER ||--o{ POST : writes
    USER ||--o{ COMMENT : makes
    USER {
        int id
        string username
        string email
        string password
        date created_at
    }
    POST ||--o{ COMMENT : has
    POST ||--o{ CATEGORY : belongs_to
    POST {
        int id
        string title
        text content
        date published_at
        bool is_published
    }
    COMMENT {
        int id
        text content
        date created_at
    }
    CATEGORY {
        int id
        string name
        string slug
    }

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Attributes and Types

You can specify attributes with their types:

Code:

erDiagram
    CUSTOMER {
        int id PK
        string name
        string email UK
        date registered_at
    }
    ORDER {
        int id PK
        int customer_id FK
        decimal total_amount
        date order_date
    }

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Relationship Labels

You can add labels to describe relationships:

Code:

erDiagram
    DOCTOR ||--o{ APPOINTMENT : "schedules"
    PATIENT ||--o{ APPOINTMENT : "books"
    APPOINTMENT ||--|| PRESCRIPTION : "results in"

Ctrl + Enter|

Additional Features

Key Types

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

Code:

erDiagram
    PRODUCT {
        int id PK
        string name UK
        decimal price
        int category_id FK
    }

Ctrl + Enter|

Styling

The appearance can be customized with:

• Different relationship types
• Attribute types
• Relationship labels
• Entity names and descriptions

Tips

• Use clear and descriptive entity names
• Include important attributes only
• Show relationships with proper cardinality
• Add meaningful relationship descriptions
• Consider normalization rules
• Group related entities together
• Use standard naming conventions