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A **one-to-many (1:N)** relationship is the **most common** type of table relationship.
It means **one record** in the first table can relate **to several records** in the second,
but each record in the second table belongs **to only one record** in the first.

### Example

**One user -> many orders**

```sql
CREATE TABLE users (
  id INT PRIMARY KEY,
  name VARCHAR(50)
);

CREATE TABLE orders (
  id INT PRIMARY KEY,
  user_id INT,
  amount DECIMAL(10,2),
  FOREIGN KEY (user_id) REFERENCES users(id)
);
```

Here:

- `users.id` is the primary key (the user's unique identifier),
- `orders.user_id` is a foreign key showing **which user the order belongs to**.

### How it looks logically

| users | orders |
|---|---|
| id = 1, name = 'Anna' | id = 101, user_id = 1, amount = 500 |
|  | id = 102, user_id = 1, amount = 900 |
| id = 2, name = 'Ivan' | id = 103, user_id = 2, amount = 200 |

Anna has **two orders**, Ivan has **one**,
but each order can belong to **only one** user.

### Example query

To join the data and see all orders with customer names:

```sql
SELECT users.name, orders.amount
FROM users
JOIN orders ON users.id = orders.user_id;
```

Result:

| name | amount |
|---|---|
| Anna | 500 |
| Anna | 900 |
| Ivan | 200 |

### Where 1:N relationships are used

- One customer -> many orders
- One instructor -> many students
- One department -> many employees
- One author -> many books

### Details

- The **parent table** (the "one" side) holds the primary key.
- The **child table** (the "many" side) holds the foreign key pointing to the "one".
- If a parent record is deleted, related rows can become "orphaned", this is controlled by settings like `ON DELETE CASCADE`, `SET NULL`, and similar.

**Summary:**
A **1:N (one-to-many)** relationship links a parent table to many dependent records.
It's used when **one object can have several dependents**,
and is implemented via a **foreign key** on the "many" table.

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