某網站包含兩個表，Customers 表和 Orders 表。編寫一個 SQL 查詢，找出所有從不訂購任何東西的客戶。

Customers 表：

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

Orders 表：

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

例如給定上述表格，你的查詢應返回：

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

解法一：

# Write your MySQL query statement below
select Name as Customers from Customers where Id not in (select CustomerId from Orders)

解法二：比解法一效率高

# Write your MySQL query statement below
select Name as Customers from Customers where
 
 not exists (select CustomerId from Orders where Orders.CustomerId = Customers.Id)

備註：

not in存在隱患，如果查詢的CustomerId為 null 時，就會查詢結果錯誤。

not exists 可避免not in上面問題

解法三：連線

# Write your MySQL query statement below
select Name as Customers from Customers as a left join Orders as b on a.Id = b.CustomerId where
 
 b.Id is null

做連線後表如下，然後做篩選。便於理解。

Id

Name

Id

CustomerId

Joe

Henry

Sam

Max