当MySQL中此类列值之一为null时，连接两列

为避免在运行查询时出现任何问题，请使用IFNULL()。让我们首先创建一个表-

mysql> create table DemoTable1793
     (
     StudentFirstName varchar(20),
     StudentLastName varchar(20)
     );

使用插入命令在表中插入一些记录-

mysql> insert into DemoTable1793 values('John','Smith');
mysql> insert into DemoTable1793 values('Carol',NULL);
mysql> insert into DemoTable1793 values(NULL,'Brown');

使用select语句显示表中的所有记录-

mysql> select * from DemoTable1793;

这将产生以下输出-

+------------------+-----------------+
| StudentFirstName | StudentLastName |
+------------------+-----------------+
| John             |           Smith |
| Carol            |            NULL |
| NULL             |           Brown |
+------------------+-----------------+
3 rows in set (0.00 sec)

这是当两个列值之一为null时连接两列的查询-

mysql> select concat(ifnull(StudentFirstName,''),ifnull(StudentLastName,'')) from DemoTable1793;

这将产生以下输出-

+----------------------------------------------------------------+
| concat(ifnull(StudentFirstName,''),ifnull(StudentLastName,'')) |
+----------------------------------------------------------------+
| JohnSmith                                                      |
| Carol                                                          |
| Brown                                                          |
+----------------------------------------------------------------+
3 rows in set (0.00 sec)