力 扣 183. Customers who never order
https://leetcode-cn.com/problems/customers-who-never-order/
A website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.
Customers 表：
+----+-------+
| Id | Name |
+----+-------+
| 1 | Joe |
| 2 | Henry |
| 3 | Sam |
| 4 | Max |
+----+-------+
Orders 表：
+ ---- + ------------ +
| Id | CustomerId |
+ ---- + ------------ +
| 1 | 3 |
| 2 | 1 |
+ ---- + ------------ +
For example, given the above table, your query should return:
+-----------+
| Customers |
+-----------+
| Henry |
| Max |
+-----------+
# Write your MySQL query statement below
#两表左连接
select Customers.name as Customers#起别名
from Customers left join Orders on Customers.id=Orders.CustomerId#左连接，条件是两表的Customers.id=Orders.CustomerId
where Orders.CustomerId is null;#查找知id是Null的is null；查找知id不是Null的is not null；is null 与 is not null
For example,
select from the User table to know that name is Null *
from user
where name is null
-------------------------------- ---
name is not null
select * from use where name is not null
