[LeetCode][MySQL] 183. Customers Who Never Order

Easy 🔗183. Customers Who Never Order

📝문제 요약

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.

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       |
+-----------+

✏️문제 풀이

SELECT

SELECT name AS Customers

• Customers
  • name : 아무것도 주문하지 않은 모든 고객의 이름

FROM

FROM Customers C LEFT JOIN Orders O ON C.id = O.customerId

• Customers 테이블과 Orders 테이블을 LEFT JOIN으로 병합
• 두 테이블의 외래키는 Customers 테이블의 id와 Orders 테이블의 customerId

WHERE

WHERE O.id IS NULL

• Orders 의 id가 NULL인 데이터만 필터링
  • 두 테이블을 병합할때 LEFT JOIN을 사용하였으므로, Orders에 존재하지 않는 데이터인 경우 NULL 값이기 때문

💯제출 코드

SELECT name AS Customers
FROM Customers C LEFT JOIN Orders O ON C.id = O.customerId
WHERE O.id IS NULL