MySQL CASE语句将自定义值放置为NULL
让我们首先创建一个表-
mysql> create table DemoTable ( FirstName varchar(20) );
使用插入命令在表中插入一些记录-
mysql> insert into DemoTable values('John');
mysql> insert into DemoTable values(NULL);
mysql> insert into DemoTable values('Adam');
mysql> insert into DemoTable values(NULL);使用select语句显示表中的所有记录-
mysql> select *from DemoTable;
这将产生以下输出-
+-----------+ | FirstName | +-----------+ | John | | NULL | | Adam | | NULL | +-----------+ 4 rows in set (0.00 sec)
现在让我们使用CASE语句-
mysql> select case when FirstName is NULL then 'UNKNOWN NAME' else FirstName end AS FirstName from DemoTable;
这将产生以下输出-
+--------------+ | FirstName | +--------------+ | John | | UNKNOWN NAME | | Adam | | UNKNOWN NAME | +--------------+ 4 rows in set (0.00 sec)
我们还可以使用IFNULL()-获得相同的结果
mysql> select IFNULL(FirstName,'UNKNOWN NAME') AS FirstName from DemoTable;
这将产生以下输出-
+--------------+ | FirstName | +--------------+ | John | | UNKNOWN NAME | | Adam | | UNKNOWN NAME | +--------------+ 4 rows in set (0.00 sec)