在MySQL中如何检查字段是否为空或null？

MySQL中的NULL和空字符串”是不同的。为了检查字段是否为空，如‘’或为null，您需要使用IS NULL属性或其他方法。您可以使用CASE语句检查所有条件。 语法如下：

SELECT *, CASE
WHEN yourColumnName =‘’ THEN ‘yourMessage1’
WHEN yourColumnName IS NULL THEN ‘yourMessage2’
ELSE CONCAT(yourColumnName,'是一个名称')
END AS anyVariableName
FROM yourTableName;

为了理解上述语法，让我们创建一个表。创建表的查询如下所示：

mysql> create table checkFieldDemo
    -> (
    -> Id int NOT NULL AUTO_INCREMENT,
    -> Name varchar(10),
    -> PRIMARY KEY(Id)
    -> );
Query OK, 0 rows affected (0.64 sec)

使用INSERT命令在表中插入一些记录。查询如下所示：

mysql> insert into checkFieldDemo(Name) values(NULL);
Query OK, 1 row affected (0.17 sec)
mysql> insert into checkFieldDemo(Name) values('John');
Query OK, 1 row affected (0.15 sec)
mysql> insert into checkFieldDemo(Name) values(NULL);
Query OK, 1 row affected (0.10 sec)
mysql> insert into checkFieldDemo(Name) values('');
Query OK, 1 row affected (0.51 sec)
mysql> insert into checkFieldDemo(Name) values('Carol');
Query OK, 1 row affected (0.14 sec)
mysql> insert into checkFieldDemo(Name) values('');
Query OK, 1 row affected (0.17 sec)
mysql> insert into checkFieldDemo(Name) values(NULL);
Query OK, 1 row affected (0.10 sec)

使用SELECT语句显示表中的所有记录。查询如下所示：

mysql> select *from checkFieldDemo;

以下是输出结果：

+----+-------+
| Id | Name  |
+----+-------+
|  1 | NULL  |
|  2 | John  |
|  3 | NULL  |
|  4 |       |
|  5 | Carol |
|  6 |       |
|  7 | NULL  |
+----+-------+
7 rows in set (0.00 sec)

以下是使用case语句检查字段是否为空或null或其他内容的查询。查询如下所示：

mysql> select *,case
    -> when Name='' then 'It is an empty string'
    -> when Name is null then 'It is a NULL value'
    -> else concat(Name,'是一个名称')
    -> END as ListOfValues
    -> from checkFieldDemo;

以下是输出结果：

+----+-------+-----------------------+
| Id | Name  | ListOfValues          |
+----+-------+-----------------------+
|  1 | NULL  | It is a NULL value    |
|  2 | John  | John is a Name        |
|  3 | NULL  | It is a NULL value    |
|  4 |       | It is an empty string |
|  5 | Carol | Carol is a Name       |
|  6 |       | It is an empty string |
|  7 | NULL  | It is a NULL value    |
+----+-------+-----------------------+
7 rows in set (0.00 sec)

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