检查MySQL中的字段是空还是空？
NULL和空字符串''在MySQL中都不相似。要检查字段是否为空值（例如''或为null），您需要使用ISNULL属性或其他方式。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 ,' is a Name') 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：
mysql> create table checkFieldDemo -> ( -> Id int NOT NULL AUTO_INCREMENT, -> Name varchar(10), -> PRIMARY KEY(Id) -> );
使用insert命令在表中插入一些记录。查询如下：
mysql> insert into checkFieldDemo(Name) values(NULL);
mysql> insert into checkFieldDemo(Name) values('John');
mysql> insert into checkFieldDemo(Name) values(NULL);
mysql> insert into checkFieldDemo(Name) values('');
mysql> insert into checkFieldDemo(Name) values('Carol');
mysql> insert into checkFieldDemo(Name) values('');
mysql> insert into checkFieldDemo(Name) values(NULL);使用select语句显示表中的所有记录。查询如下：
mysql> select *from checkFieldDemo;
以下是输出：
+----+-------+ | Id | Name | +----+-------+ | 1 | NULL | | 2 | John | | 3 | NULL | | 4 | | | 5 | Carol | | 6 | | | 7 | NULL | +----+-------+ 7 rows in set (0.00 sec)
这是查询要检查的字段为空还是null或其他使用case语句的查询。查询如下：
mysql> select *,case -> when Name='' then 'It is an empty string' -> when Name is null then 'It is a NULL value' -> else concat(Name,' is a Name') -> END as ListOfValues -> from checkFieldDemo;
以下是输出：
+----+-------+-----------------------+ | 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)