MySQL查詢不會返回所需的結果。 沒有訂單的客戶不顯示

Question

我仍在進行SQL培訓。 所以我有兩個表CUSTOMERS和ORDERS（外鍵表）。 現在，我需要獲得所有沒有任何訂單的客戶。 這是我的查詢

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

但是我確實收到了空集（0秒）/但是我不確定那些存在。

Answer 1

我懷疑ORDERS表的ID_CUSTOMER列中有一些NULL值。 當子查詢中存在NULL值時， NOT IN失敗。

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 );