# 183. Customers Who Never Order

> Nguồn: https://tiennhm.io.vn/docs/leetcode/database/183-customers-who-never-order
> Write a solution to find all customers who never order anything.

## Giới thiệu bài toán

Chi tiết: https://leetcode.com/problems/customers-who-never-order/

Table: `Customers`

| Column Name | Type    |
| ----------- | ------- |
| id          | int     |
| name        | varchar |

- `id` is the primary key (column with unique values) for this table.
- Each row of this table indicates the ID and name of a customer.

Table: `Orders`

| Column Name | Type |
| ----------- | ---- |
| id          | int  |
| customerId  | int  |

- `id` is the primary key (column with unique values) for this table.
- `customerId` is a foreign key (reference columns) of the ID from the `Customers` table.
- Each row of this table indicates the ID of an order and the ID of the customer who ordered it.

> **Tip: Yêu cầu**
>
> Write a solution to find all customers who never order anything.
>
> Return the result table in any order. The result format is in the following example.
Example 1:

Input: `Customers` table:

| id | name  |
| -- | ----- |
| 1  | Joe   |
| 2  | Henry |
| 3  | Sam   |
| 4  | Max   |

`Orders` table:

| id | customerId |
| -- | ---------- |
| 1  | 3          |
| 2  | 1          |

Output:

| Customers |
| --------- |
| Henry     |
| Max       |

## Giải quyết bài toán

```sql
SELECT name Customers
FROM Customers
WHERE id NOT IN (SELECT customerId FROM Orders);
```

Tham khảo: https://leetcode.com/submissions/detail/1039107765/