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.

Entity Relationship Diagram (ERD)

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.

@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

2. Order System

This represents tables and their foreign key constraints.

@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

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