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leetcode 183: Customers Who Never Order
题目描述：
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 | +-----------+
题目大意：
假设一个网站包含两个表， 顾客表Customers和订单表Orders。编写一个SQL查询找出所有从未下过订单的顾客。
解题思路：
使用NOT IN，NOT EXISTS，或者LEFT JOIN均可。
SQL语句：
使用NOT IN（Accepted 777ms）：
# Write your MySQL query statement below SELECT Name FROM Customers c WHERE c.Id NOT IN (SELECT CustomerId FROM Orders o);
使用NOT EXISTS（Accepted 745ms）：
# Write your MySQL query statement below SELECT Name FROM Customers c WHERE NOT EXISTS (SELECT CustomerId FROM Orders o WHERE o.CustomerId = c.id);
使用LEFT JOIN（Accepted 726ms）：
# Write your MySQL query statement below SELECT Name FROM Customers c LEFT JOIN Orders o ON c.Id = o.CustomerId WHERE o.Id IS NULL;
leetcode 183: Customers Who Never Order
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