題目8：MySQL----------Duplicate Emails，email

題目8：MySQL----------Duplicate Emails，email

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

題目解答：

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

另外還可以寫成下面兩種方式：

SELECT A.Name from Customers AWHERE NOT EXISTS (SELECT 1 FROM Orders B WHERE A.Id = B.CustomerId)SELECT A.Name from Customers ALEFT JOIN Orders B on  a.Id = B.CustomerIdWHERE b.CustomerId is NULL