MySQL 查询不返回所需的结果。没有任何订单的客户，不显示

Question

我仍在进行 SQL 培训。所以我有两个表 CUSTOMERS 和 ORDERS(外键表)。现在我需要让所有客户都没有任何订单。这是我的查询

select LASTNAME, FIRSTNAME 
  from CUSTOMERS 
  WHERE CUSTOMERS.ID NOT IN 
      (select ID_CUSTOMER from ORDERS);

+-----------+--------------+------+-----+---------+-------+
| Field     | Type         | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+-------+
| ID        | int(11)      | NO   | PRI | NULL    |       |
| FIRSTNAME | varchar(50)  | YES  |     | NULL    |       |
| LASTNAME  | varchar(50)  | YES  |     | NULL    |       |
| ADDRESS   | varchar(100) | YES  |     | NULL    |       |
+-----------+--------------+------+-----+---------+-------+


+---------------+--------------+------+-----+---------+-------+
| Field         | Type         | Null | Key | Default | Extra |
+---------------+--------------+------+-----+---------+-------+
| ID            | int(11)      | NO   | PRI | NULL    |       |
| PRODUCT_NAME  | varchar(100) | YES  |     | NULL    |       |
| PRODUCT_PRICE | double(10,4) | YES  |     | NULL    |       |
| DATE_ORDER    | date         | YES  |     | NULL    |       |
| ID_CUSTOMER   | int(11)      | YES  |     | NULL    |       |
| AMOUNT        | int(11)      | YES  |     | NULL    |       |
+---------------+--------------+------+-----+---------+-------+

但我确实收到了空集(0,00 秒)/但我确定这些都存在；

Answer 1

我怀疑 ORDERS 表的 ID_CUSTOMER 列中有一些 NULL 值。当子查询中存在 NULL 值时，NOT IN 失败。

使用NOT EXISTS

select LASTNAME, FIRSTNAME 
  from CUSTOMERS C
  WHERE NOT EXISTS (select ID_CUSTOMER from ORDERS O Where C.ID = O.ID_CUSTOMER );