我的数据库中有以下四个表：
+--------------+-------------+------+-----+---------+----------------+
| 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 | |
+------------+-------------+------+-----+---------+----------------+
我正在尝试获取格式如下的结果（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，如果设备的 pnpnum 为空，我将无法包含该设备。
+------------+-------------+-----------------------+
| devices_id | description | pnpnum |
+------------+-------------+-----------------------+
| 1 | ex | 1234 |
| 2 | ex2 | 2345 |
| 4 | ex4 | 3456, 4567, 5678, 6879|
+------------+-------------+-----------------------+
我的 SQL 语句中缺少什么允许我包含空值？