更改此查询行：

CONCAT_WS(',',IFNULL(a.PD_right, ''),IFNULL(a.PD_left, '')) as PD_RL

到

CONCAT_WS(',',IFNULL(a.PD_right, 0),IFNULL(a.PD_left, 0)) as PD_RL

或者，您可以在连接之前测试这些值 CASE ...WHEN...THEN...END 语法。
更多信息请看这里。

如果我理解正确，只需将空字符串替换为“0”，如 CONCAT_WS(',',IFNULL(a.PD_right, '0'),IFNULL(a.PD_left, '0')) as PD_RL 应该有用。
如果要检测null或空字符串“”，请改用if函数。
我在我的机器上做了一个简单的测试，输出如下：

$ mysql -u root
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 9
Server version: 8.0.12 Homebrew

Copyright (c) 2000, 2018, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

(root@localhost) [(none)]> SET @left = 3, @right = 4;
Query OK, 0 rows affected (0.00 sec)

(root@localhost) [(none)]> SELECT CONCAT_WS(',', IF(@left IS NULL OR @left = '', '0', @left), IF(@right IS NULL OR @right = '', '0', @right)) as PD_RL;
+-------+
| PD_RL |
+-------+
| 3,4   |
+-------+
1 row in set (0.00 sec)

(root@localhost) [(none)]> SET @left = 3, @right = NULL;
Query OK, 0 rows affected (0.00 sec)

(root@localhost) [(none)]> SELECT CONCAT_WS(',', IF(@left IS NULL OR @left = '', '0', @left), IF(@right IS NULL OR @right = '', '0', @right)) as PD_RL;
+-------+
| PD_RL |
+-------+
| 3,0   |
+-------+
1 row in set (0.00 sec)

(root@localhost) [(none)]> SET @left = NULL, @right = 4;
Query OK, 0 rows affected (0.00 sec)

(root@localhost) [(none)]> SELECT CONCAT_WS(',', IF(@left IS NULL OR @left = '', '0', @left), IF(@right IS NULL OR @right = '', '0', @right)) as PD_RL;
+-------+
| PD_RL |
+-------+
| 0,4   |
+-------+
1 row in set (0.00 sec)

(root@localhost) [(none)]> SET @left = NULL, @right = NULL;
Query OK, 0 rows affected (0.00 sec)

(root@localhost) [(none)]> SELECT CONCAT_WS(',', IF(@left IS NULL OR @left = '', '0', @left), IF(@right IS NULL OR @right = '', '0', @right)) as PD_RL;
+-------+
| PD_RL |
+-------+
| 0,0   |
+-------+
1 row in set (0.00 sec)

(root@localhost) [(none)]> SET @left = '', @right = '';
Query OK, 0 rows affected (0.00 sec)

(root@localhost) [(none)]> SELECT CONCAT_WS(',', IF(@left IS NULL OR @left = '', '0', @left), IF(@right IS NULL OR @right = '', '0', @right)) as PD_RL;
+-------+
| PD_RL |
+-------+
| 0,0   |
+-------+
1 row in set (0.00 sec)