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