Question

Formatted question description: https://leetcode.ca/all/183.html

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

Algorithm

keyword NOT IN or Not exists

Code

SQL

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

--

SELECT Name AS Customers FROM Customers c
WHERE NOT EXISTS (SELECT * FROM Orders o WHERE o.CustomerId = c.Id);

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