CASE WHEN column1 IS NULL THEN NULL ELSE column2 END with MySQL
mysqlmysqli database
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 -> ); Query OK, 0 rows affected (0.72 sec)
使用插入命令在表中插入一些记录 −
mysql> insert into DemoTable values('Chris',45,null);
Query OK, 1 row affected (0.14 sec)
mysql> insert into DemoTable values('David',null,78);
Query OK, 1 row affected (0.16 sec)
mysql> insert into DemoTable values('Bob',67,98);
Query OK, 1 row affected (0.14 sec)
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)
以下是实现 CASE WHEN 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)
