Leetcode5(SQL Part)【谁没点餐】

原创已于 2023-09-14 16:48:34 修改 · 351 阅读

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#Customers Who Never Order

于 2018-09-05 15:43:09 首次发布

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本文介绍了一种SQL查询技巧，用于找出从未下过订单的客户。通过几种不同的SQL语句实现这一目标，包括使用NOT EXISTS、NOT IN及LEFT JOIN等方法。

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

这个其实和上面的一个题是类似的，只不过有取反的意思：下面是我做的：

Select Name as Customers from Customers c 
where not exists (Select CustomerId from Orders o where o.CustomerId=c.Id);

再看看大神的：

select Name as 'Customers' from Customers where id not in (select CustomerId from orders)

直接看ID，很可以。
其他三种：

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

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