[LeetCode] LEFT JOIN : 183. Customers Who Never Order

1. 문제

 

Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.

 

Table: Customers.

+----+-------+

| Id | Name |

+----+-------+

| 1  | Joe |

| 2  | Henry |

| 3  | Sam |

| 4  | Max |

+----+-------+

 

Table: Orders.

+----+------------+

| Id | CustomerId |

+----+------------+

| 1 | 3 |

| 2 | 1 |

+----+------------+

 

Using the above tables as example, return the following:

+-----------+

| Customers |

+-----------+

| Henry |

| Max |

+-----------+

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2. 답

SELECT c.name AS Customers
FROM customers AS c
    LEFT JOIN orders AS o ON c.id = o.customerid
WHERE o.id IS NULL

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3. KEY POINT

 

• LEFT JOIN : 왼쪽 테이블 기준으로 합치기
• IS NULL : NULL값 인지 확인, True 일 경우 조건 해당됨
• AS : 별명 붙이기

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Customers Who Never Order - LeetCode