【sql】統計沒有預約的客戶 Customers Who Never Order

問題：

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

① 使用NOT IN，找沒有在Orders表中出現的顧客Id。769 ms

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

② 使用NOT EXISTS。795 ms

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

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

③ 用左交來聯合兩個表，只要找出右邊的CustomerId爲Null的顧客就是沒有下單的。795ms

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