如果使用 php/mysqli 值为空，如何将列值设置为零？

Question

连接两列时（3,4）。如果第一个值为空或 null，则为零（0,4），如果第二个值为空或 null，则为零，如 3,0。如何在 sql 查询中使用php.

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

在上面的代码中，连接PD_right和PD_left。

现在越来越喜欢了

1. 如果两个值为空，则显示为 (,) 仅逗号。

2. 如果有两个值，它将显示为 3,4

3. 如果第一个值为空，它将显示为 ,4

4. 如果第二个值为空，它将显示为 3，

如何解决这些问题？我的数据库列是空白的。它不是 NULL 或 null

Answer 1

如果我理解正确，只需像 CONCAT_WS(',',IFNULL(a.PD_right, '0'),IFNULL(a.PD_left, '0')) as PD_RL 一样将空字符串替换为 '0' 即可。

如果您想检测 NULL 或空字符串 ''，请尝试使用 IF 函数。

我在我的机器上做了简单的测试，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)

Answer 2

更改这行查询：

  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，您可以使用CASE ...WHEN...THEN...END 语法在连接之前测试这些值。

查看更多here。