心得

這題要show出沒有訂過東西的客戶，所以可以使用LEFT JOIN來關聯兩張表。

問題

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

答案

1. LEFT JOIN

   # Write your MySQL query statement below
   SELECT      a.Name AS Customers
   FROM        Customers AS a
   LEFT JOIN   Orders AS b
   ON          a.Id = b.CustomerId
   WHERE       b.CustomerId IS NULL

2. Sub Query

   # Write your MySQL query statement below
   SELECT  a.Name AS Customers
   FROM    Customers AS a
   WHERE   (   SELECT  COUNT(1)
               FROM    Orders AS z
               WHERE   z.CustomerId = a.Id) = 0