sql notes: leetcode#183 customers who never order

Problem

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.

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+----+-------+
| Id | Name  |
+----+-------+
| 1  | Joe   |
| 2  | Henry |
| 3  | Sam   |
| 4  | Max   |
+----+-------+

Table: Orders.

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+----+------------+
| Id | CustomerId |
+----+------------+
| 1  | 3          |
| 2  | 1          |
+----+------------+

Using the above tables as example, return the following:

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

Analysis

We can output customers whose id’s are not listed in the CustomerId of table Orders:

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SELECT Name AS Customers FROM Customers WHERE Id NOT IN
(SELECT CustomerId FROM Orders);

Also, we can left join Customers table with Orders table. Then output customers whose order id is null:

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SELECT Name AS Customers FROM Customers AS c
LEFT JOIN Orders AS o ON c.Id = o.CustomerId
WHERE o.Id IS NULL;

Solution

Solution 1

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SELECT Name AS Customers FROM Customers WHERE Id NOT IN
(SELECT CustomerId FROM Orders);

Solution 2

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SELECT Name AS Customers FROM Customers AS c
LEFT JOIN Orders AS o ON c.Id = o.CustomerId
WHERE o.Id IS NULL;

Link

183. Customers Who Never Order
(中文版) SQL 笔记: Leetcode#183 Customers Who Never Order