题目7：MySQL----------Customers Who Never Order

​

​Customers​

​ table and the ​

​Orders​

​​

​Customers​

​.

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

| Id | Name |

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

| 1 | Joe |

| 2 | Henry |

| 3 | Sam |

| 4 | Max |

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

​

​Orders​

​.

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

| Id | CustomerId |

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

| 1 | 3 |

| 2 | 1 |

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

Using the above tables as example, return the following:

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

| Customers |

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

| Henry |

| Max |

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

题目解答：

SELECT A.Name from Customers A
WHERE A.Id NOT IN (SELECT B.CustomerId from Orders B)      

另外还可以写成下面两种方式：

SELECT A.Name from Customers A
WHERE NOT EXISTS (SELECT 1 FROM Orders B WHERE A.Id = B.CustomerId)

SELECT A.Name from Customers A
LEFT JOIN Orders B on  a.Id = B.CustomerId
WHERE b.CustomerId is NULL