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entity-relationship.md

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Entity relationship diagrams cheatsheet

Official documentation.

Example

erDiagram
    User {
        Int id PK
        String username
        Int serverId FK
    }

    Server {
        Int id PK
        String serverName
    }

    Server ||--o{ User : has
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erDiagram
    User {
        Int id PK
        String username
        Int serverId FK
    }

    Server {
        Int id PK
        String serverName
    }

    Server ||--o{ User : has

Defining entities

erDiagram
    User {
        String Username PK "The user's name, a primary key"
        Date dateCreated "When the user was created"
        Int Server FK "The user's server, a foreign key"
    }
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erDiagram
    User {
        String Username PK "The user's name, a primary key"
        Date dateCreated "When the user was created"
        Int Server FK "The user's server, a foreign key"
    }

Defining relationships

Numerical relationship

erDiagram
    User1 |o--o| Item1 : "Zero or one - Zero or one"
    User2 }o--o{ Item2 : "Zero or more - Zero or more"
    User3 ||--|| Item3 : "Exactly one - Exactly one"
    User4 }|--|{ Item4 : "One or more - One or more"
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erDiagram
    User1 |o--o| Item1 : "Zero or one - Zero or one"
    User2 }o--o{ Item2 : "Zero or more - Zero or more"
    User3 ||--|| Item3 : "Exactly one - Exactly one"
    User4 }|--|{ Item4 : "One or more - One or more"

Identifying relationship

erDiagram
    AppleTree ||..|| Apple : "Reliant entity"
    Flower ||--|| Leaf : "Independent entities"
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erDiagram
    AppleTree ||..|| Apple : "Reliant entity"
    Flower ||--|| Leaf : "Independent entities"