标题8：MySQL-Duplicate Emails

题目8：MySQL----------Duplicate Emails

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

3楼mchdba昨天 22:31: mysqlu0026gt; CREATE TABLE Customers SELECT 1 AS id,u0026#39;Joeu0026#39; AS NAME UNION ALL SELECT 2,u0026#39;Henryu0026#39; UNION ALL SELECT 3,u0026#39;Samu0026#39; UNION ALL SELECT 4,u0026#39;Maxu0026#39;;nQuery OK, 4 rows affected (0.24 sec)nRecords: 4 Duplicates: 0 Warnings: 0nnmysqlu0026gt;nmysqlu0026gt; select * from Customers;n+----+-------+n| id | NAME |n+----+-------+n| 1 | Joe |n| 2 | Henry |n| 3 | Sam |n| 4 | Max |n+----+-------+n4 rows in set (0.02 sec)nnmysqlu0026gt; nmysqlu0026gt; CREATE TABLE Orders SELECT 1 AS id,3 AS CustomerId UNION ALL SELECT 2,1;nQuery OK, 2 rows affected (0.08 sec)nRecords: 2 Duplicates: 0 Warnings: 0nnmysqlu0026gt;nnmysqlu0026gt; select * from Orders;n+----+------------+n| id | CustomerId |n+----+------------+n| 1 | 3 |n| 2 | 1 |n+----+------------+n2 rows in set (0.00 sec)nnmysqlu0026gt;

2楼mchdba昨天 22:31: mysqlu0026gt; SELECT A.Name FROM Customers A n -u0026gt; LEFT JOIN Orders B ON a.Id = B.CustomerId n -u0026gt; WHERE b.CustomerId IS NULL ;nERROR 1054 (42S22): Unknown column u0026#39;b.CustomerIdu0026#39; in u0026#39;where clauseu0026#39;nmysqlu0026gt; nn为什么报错？n对了，是表的别名区分大小写了，所以执行报错了。

1楼mchdba昨天 22:31: 写成：nmysqlu0026gt; SELECT A.Name FROM Customers A n -u0026gt; LEFT JOIN Orders B ON A.Id = B.CustomerId n -u0026gt; WHERE B.CustomerId IS NULL ;n+-------+n| Name |n+-------+n| Henry |n| Max |n+-------+n2 rows in set (0.04 sec)nnmysqlu0026gt; nn就OK了。

标题8：MySQL-Duplicate Emails

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