> ## Documentation Index
> Fetch the complete documentation index at: https://mintlify.com/mermaid-js/mermaid/llms.txt
> Use this file to discover all available pages before exploring further.

# Entity Relationship diagrams

> Model data structures and relationships between entities in a domain

An entity-relationship (ER) model describes interrelated things of interest in a specific domain of knowledge. An ER model is composed of entity types and specifies relationships that can exist between entities.

## Basic example

```mermaid theme={null}
---
title: Order example
---
erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    CUSTOMER }|..|{ DELIVERY-ADDRESS : uses
```

<Note>
  Entity names are often capitalized, although this is not required in Mermaid.
</Note>

## Entities with attributes

Define attributes with their types:

```mermaid theme={null}
erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string name
        string custNumber
        string sector
    }
    ORDER ||--|{ LINE-ITEM : contains
    ORDER {
        int orderNumber
        string deliveryAddress
    }
    LINE-ITEM {
        string productCode
        int quantity
        float pricePerUnit
    }
```

## Syntax

### Entities and relationships

Each statement consists of:

```
<first-entity> [<relationship> <second-entity> : <relationship-label>]
```

Example:

```mermaid theme={null}
erDiagram
    PROPERTY ||--|{ ROOM : contains
```

This reads as: "A property contains one or more rooms, and a room is part of one and only one property."

### Relationship syntax

The relationship consists of three parts:

* Cardinality of the first entity
* Whether the relationship confers identity
* Cardinality of the second entity

#### Cardinality

```mermaid theme={null}
erDiagram
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string name
    }
    ORDER {
        int orderNumber
    }
```

<Accordion title="Cardinality values">
  | Value (left) | Value (right) | Meaning                       |
  | :----------: | :-----------: | ----------------------------- |
  |     `\|o`    |     `o\|`     | Zero or one                   |
  |    `\|\|`    |     `\|\|`    | Exactly one                   |
  |     `}o`     |      `o{`     | Zero or more (no upper limit) |
  |     `}\|`    |     `\|{`     | One or more (no upper limit)  |

  Aliases like `one or more`, `zero or many`, `1+`, `0+` are also supported.
</Accordion>

### Identification

Relationships can be identifying (solid line) or non-identifying (dashed line):

```mermaid theme={null}
erDiagram
    CAR ||--o{ NAMED-DRIVER : allows
    PERSON }o..o{ NAMED-DRIVER : is
```

Using aliases:

```mermaid theme={null}
erDiagram
    CAR 1 to zero or more NAMED-DRIVER : allows
    PERSON many(0) optionally to 0+ NAMED-DRIVER : is
```

## Attributes

Define attributes with type and name:

```mermaid theme={null}
erDiagram
    CAR ||--o{ NAMED-DRIVER : allows
    CAR {
        string registrationNumber
        string make
        string model
    }
    PERSON ||--o{ NAMED-DRIVER : is
    PERSON {
        string firstName
        string lastName
        int age
    }
```

### Attribute keys and comments

Specify primary keys, foreign keys, and comments:

```mermaid theme={null}
erDiagram
    CAR ||--o{ NAMED-DRIVER : allows
    CAR {
        string registrationNumber PK
        string make
        string model
        string[] parts
    }
    PERSON ||--o{ NAMED-DRIVER : is
    PERSON {
        string driversLicense PK "The license #"
        string(99) firstName "Only 99 characters are allowed"
        string lastName
        string phone UK
        int age
    }
    NAMED-DRIVER {
        string carRegistrationNumber PK, FK
        string driverLicence PK, FK
    }
    MANUFACTURER only one to zero or more CAR : makes
```

<Tip>
  Available keys: `PK` (Primary Key), `FK` (Foreign Key), `UK` (Unique Key). Multiple keys can be specified with commas.
</Tip>

## Entity name aliases

Use aliases for display names:

```mermaid theme={null}
erDiagram
    p[Person] {
        string firstName
        string lastName
    }
    a["Customer Account"] {
        string email
    }
    p ||--o| a : has
```

## Unicode and Markdown

### Unicode text

```mermaid theme={null}
erDiagram
    "This ❤ Unicode"
```

### Markdown formatting

```mermaid theme={null}
erDiagram
    "This **is** _Markdown_"
```

## Direction

Set the diagram orientation:

### Top to bottom

```mermaid theme={null}
erDiagram
    direction TB
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string name
        string custNumber
        string sector
    }
    ORDER ||--|{ LINE-ITEM : contains
    ORDER {
        int orderNumber
        string deliveryAddress
    }
    LINE-ITEM {
        string productCode
        int quantity
        float pricePerUnit
    }
```

### Left to right

```mermaid theme={null}
erDiagram
    direction LR
    CUSTOMER ||--o{ ORDER : places
    CUSTOMER {
        string name
    }
    ORDER {
        int orderNumber
    }
```

<Accordion title="Direction options">
  * `TB` - Top to bottom
  * `BT` - Bottom to top
  * `RL` - Right to left
  * `LR` - Left to right
</Accordion>

## Styling

### Styling a node

```mermaid theme={null}
erDiagram
    id1||--||id2 : label
    style id1 fill:#f9f,stroke:#333,stroke-width:4px
    style id2 fill:#bbf,stroke:#f66,stroke-width:2px,color:#fff,stroke-dasharray: 5 5
```

### Using classes

```mermaid theme={null}
erDiagram
    direction TB
    CAR:::someclass {
        string registrationNumber
        string make
        string model
    }
    PERSON:::someclass {
        string firstName
        string lastName
        int age
    }
    HOUSE:::someclass

    classDef someclass fill:#f96
```

With relationships:

```mermaid theme={null}
erDiagram
    CAR {
        string registrationNumber
        string make
        string model
    }
    PERSON {
        string firstName
        string lastName
        int age
    }
    PERSON:::foo ||--|| CAR : owns
    PERSON o{--|| HOUSE:::bar : has

    classDef foo stroke:#f00
    classDef bar stroke:#0f0
    classDef foobar stroke:#00f
```

## Configuration

### Layout

For larger diagrams, use ELK layout:

```mermaid theme={null}
---
title: Order example
config:
    layout: elk
---
erDiagram
    CUSTOMER ||--o{ ORDER : places
    ORDER ||--|{ LINE-ITEM : contains
    CUSTOMER }|..|{ DELIVERY-ADDRESS : uses
```

<Note>
  Requires Mermaid version 9.4+ with lazy-loading enabled.
</Note>