One-to-Many Relationship

Introduction

1:N is the most common relationship — one parent row has many children. One user has many orders, one author has many books, one team has many members.

Understanding the topic

Core concepts to understand:

• FK on the child side: orders.user_id REFERENCES users(id).
• No UNIQUE on the FK — many children allowed.
• Always index the child's FK column.
• Listing children: WHERE user_id = ?.
• Deleting parent: choose CASCADE / RESTRICT.

Syntax reference

Visual workflow / architecture:

users                 orders
┌────┐  1:N  FK    ┌──────────┐
│ id │ ◀───────── │ user_id  │
└────┘            │ total    │
                  │ created  │
                  └──────────┘  many rows per user

Real-world use

Every customer-orders, post-comments, channel-messages and account-transactions schema is 1:N.

Best practices

• Index the FK column always.
• Pick a delete strategy explicitly.
• Don't denormalize counts unless you measure a need.

Hands-on exercise

Interview preparation — practice these questions:

• Q1. Where does the FK live in 1:N?
• Q2. Why index that column?
• Q3. Two patterns to count children quickly.
• Q4. CASCADE vs RESTRICT trade-offs.
• Q5. Examples of 1:N from your last project.