CONCAT()如果函数的任何参数为NULL ，它将返回NULL。这意味着如果我们将包含NULL值的列名作为CONCAT()函数的参数之一传递，则MySQL将返回NULL 。以下是“学生”表的示例进行说明。
示例
我们串联了两个字符串的值，并且在第5行第一处，该值为NULL，因此串联结果也为NULL。
mysql> Select Name, Address, CONCAT(Name,' Resident of ',Address)AS 'Detail of Student' from Student;
+---------+---------+---------------------------+
| Name | Address | Detail of Student |
+---------+---------+---------------------------+
| Gaurav | Delhi | Gaurav Resident of Delhi |
| Aarav | Mumbai | Aarav Resident of Mumbai |
| Harshit | Delhi | Harshit Resident of Delhi |
| Gaurav | Jaipur | Gaurav Resident of Jaipur |
| Yashraj | NULL | NULL |
+---------+---------+---------------------------+
5 rows in set (0.00 sec)正文完