当MySQL的column1为NULL则为NULL ELSE column2结束时的情况
为此，您可以使用CASE语句。让我们首先创建一个表-
mysql> create table DemoTable -> ( -> Name varchar(20), -> Marks1 int, -> Marks2 int -> );
使用插入命令在表中插入一些记录-
mysql> insert into DemoTable values('Chris',45,null);
mysql> insert into DemoTable values('David',null,78);
mysql> insert into DemoTable values('Bob',67,98);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-
+-------+--------+--------+ | Name | Marks1 | Marks2 | +-------+--------+--------+ | Chris | 45 | NULL | | David | NULL | 78 | | Bob | 67 | 98 | +-------+--------+--------+ 3 rows in set (0.00 sec)
这是实现CASE的查询-
mysql> select *, -> (case when Marks1 is null then null else Marks2 end ) as Value -> from DemoTable;
这将产生以下输出-
+-------+--------+--------+-------+ | Name | Marks1 | Marks2 | Value | +-------+--------+--------+-------+ | Chris | 45 | NULL | NULL | | David | NULL | 78 | NULL | | Bob | 67 | 98 | 98 | +-------+--------+--------+-------+ 3 rows in set (0.00 sec)