LeetCode：183.从不订购的客户
题目链接：https://leetcode-cn.com/problems/customers-who-never-order/
题目
某网站包含两个表 Customers 表和 Orders 表。编写一个 SQL 查询，找出所有从不订购任何东西的客户。
Customers 表：
+----+-------+
| Id | Name |
+----+-------+
| 1 | Joe |
| 2 | Henry |
| 3 | Sam |
| 4 | Max |
+----+-------+
Orders 表：
+----+------------+
| Id | CustomerId |
+----+------------+
| 1 | 3 |
| 2 | 1 |
+----+------------+
例如给定上述表格，你的查询应返回：
+-----------+
| Customers |
+-----------+
| Henry |
| Max |
+-----------+
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/customers-who-never-order
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解答
通过 not exists 实现。
---- oracle ----
/* Write your PL/SQL query statement below */
select a.Name as Customers
from Customers a
where not exists(select 1 from Orders b where a.Id = b.CustomerId) ---- 909ms
通过 left join 实现。
---- oracle ----
/* Write your PL/SQL query statement below */
select a.Name as Customers
from Customers a
left join Orders b
on a.Id = b.CustomerId
where b.Id is null ---- 1246ms
通过子查询和 not in 实现。
---- oracle ----
/* Write your PL/SQL query statement below */
select a.Name as Customers
from Customers a
where Id not in (select CustomerId from Orders) ---- 1172ms
思考
- 通过
not exists实现，效率最高； - 通过
left join关联之后为空实现；
