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

1. 通过 not exists 实现，效率最高；
2. 通过 left join 关联之后为空实现；