包含在SQL查询中的空值

Question

，我有以下四个表在我的数据库：

+--------------+-------------+------+-----+---------+----------------+ 
| Field  | Type  | Null | Key | Default | Extra   | 
+--------------+-------------+------+-----+---------+----------------+ 
| complex_id | int(11)  | NO | PRI | NULL | auto_increment | 
| complex_name | varchar(45) | NO |  | NULL |    | 
+--------------+-------------+------+-----+---------+----------------+ 


+--------------+-------------+------+-----+---------+----------------+ 
| Field  | Type  | Null | Key | Default | Extra   | 
+--------------+-------------+------+-----+---------+----------------+ 
| machine_id | int(11)  | NO | PRI | NULL | auto_increment | 
| complex_id | int(11)  | NO | MUL | NULL |    | 
| machine_name | varchar(45) | NO |  | NULL |    | 
+--------------+-------------+------+-----+---------+----------------+ 


+-----------------+--------------+------+-----+---------+----------------+ 
| Field   | Type   | Null | Key | Default | Extra   | 
+-----------------+--------------+------+-----+---------+----------------+ 
| devices_id  | int(11)  | NO | PRI | NULL | auto_increment | 
| machine_id  | int(11)  | NO | MUL | NULL |    | 
| description  | varchar(255) | NO |  | NULL |    | 
| location  | varchar(255) | YES |  | NULL |    | 
| verification | varchar(255) | YES |  | NULL |    | 
| rack_num  | varchar(8) | YES |  | NULL |    | 
| section_num  | varchar(8) | YES |  | NULL |    | 
| color_or_number | varchar(16) | YES |  | NULL |    | 
| normal_position | varchar(16) | YES |  | NULL |    | 
+-----------------+--------------+------+-----+---------+----------------+ 


+------------+-------------+------+-----+---------+----------------+ 
| Field  | Type  | Null | Key | Default | Extra   | 
+------------+-------------+------+-----+---------+----------------+ 
| pnp_id  | int(11)  | NO | PRI | NULL | auto_increment | 
| devices_id | int(11)  | NO | MUL | NULL |    | 
| pnp_num | varchar(45) | NO |  | NULL |    | 
+------------+-------------+------+-----+---------+----------------+ 

我试图让格式如下结果（NULL值显示为空白）：

+------------+-------------+-----------------------+ 
| devices_id | description | pnpnum    | 
+------------+-------------+-----------------------+ 
| 1   | ex   | 1234     | 
| 2   | ex2   | 2345     | 
| 3   | ex3   |      | 
| 4   | ex4   | 3456, 4567, 5678, 6879| 
+------------+-------------+-----------------------+ 

使用以下SQL查询，

SELECT *, GROUP_CONCAT(pnp.pnp_num separator ', ') pnpnum 
FROM devices 
JOIN pnp ON devices.devices_id = pnp.devices_id 
WHERE devices.machine_id = 1 
GROUP BY devices.devices_ID 
ORDER BY devices.description; 

我的结果比较接近，但是，我无法包含设备if它有一个空pnpnum。

+------------+-------------+-----------------------+ 
| devices_id | description | pnpnum    | 
+------------+-------------+-----------------------+ 
| 1   | ex   | 1234     | 
| 2   | ex2   | 2345     | 
| 4   | ex4   | 3456, 4567, 5678, 6879| 
+------------+-------------+-----------------------+ 

什么是我从我的SQL语句，将允许我包含空值缺少？

Answer 1

您需要使用LEFT JOIN，因为即使没有匹配，它也会从左表中返回所有结果，而将右表null中的字段留下。