> For the complete documentation index, see [llms.txt](https://www.pranaypourkar.co.in/the-programmers-guide/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://www.pranaypourkar.co.in/the-programmers-guide/system-design/system-design-methodology/diagrams/uml-diagrams/plantuml/er-diagram-entity-relationship.md).

# ER Diagram (Entity-Relationship)

## About

An ER Diagram represents data models and relationships between entities in a database. It is widely used in database design and schema planning.

{% hint style="success" %}
Refer to the official documentation for more details - <https://plantuml.com/er-diagram>
{% endhint %}

{% content-ref url="/pages/45bclIZBVrOBtVlTGEod" %}
[Entity Relationship Diagram (ERD)](/the-programmers-guide/database/sql-databases/rdbms/entity-relationship-diagram-erd.md)
{% endcontent-ref %}

### **Key Elements**

1. **Entities**
   * Represented as **rectangles**, denoting **real-world objects** (e.g., `Customer`, `Order`).
2. **Attributes**
   * Represented as **ellipses** connected to entities.
   * **Primary Key (PK)** – Unique identifier (underlined).
   * **Foreign Key (FK)** – Connects to another entity.
3. **Relationships**
   * Represented as **diamonds** between entities (e.g., `Places Order`).
   * **1:1, 1:M, M:N** relationships specify cardinality.
4. **Cardinality & Participation**
   * **(1,1), (0,N), (M,N)** define relationship constraints.

## 1. **Employee Management System**

This represents **database entities and relationships** for an **Employee Management System**.

{% hint style="success" %}

* **Entities (`entity Name { }`)**.
* **Primary Keys (`* field : type`)**.
* **Relationships (`}|..||`)**.
  {% endhint %}

```java
@startuml
title Employee Management - ER Diagram

entity Employee {
    * emp_id : INT
    --
    name : STRING
    age : INT
    department_id : INT
}

entity Department {
    * dept_id : INT
    --
    dept_name : STRING
}

Employee }|..|| Department : belongs_to

@enduml
```

<figure><img src="/files/xyshSxdpE5X1fe9C6QXO" alt="" width="245"><figcaption></figcaption></figure>

## 2. Order System

This represents **tables and their foreign key constraints**.

```plant-uml
@startuml
title Database Schema - Order System

entity Customers {
    * id : INT
    --
    name : STRING
    email : STRING
}

entity Orders {
    * id : INT
    --
    customer_id : INT
    total_amount : DECIMAL
}

entity Order_Items {
    * id : INT
    --
    order_id : INT
    product_id : INT
    quantity : INT
}

Customers ||--|{ Orders : has
Orders ||--|{ Order_Items : contains

@enduml
```

<figure><img src="/files/3REUNBG3D0PncVHBhNvz" alt="" width="227"><figcaption></figcaption></figure>


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