mysql 字段添加以及删除

mysql> select * from user;
+------+-----------+-----------+
| id   | name      | address   |
+------+-----------+-----------+
|     | xiaoming  | beijing   |
|     | xiaobai   | shandong  |
|     | xiaohong  | suzhou    |
|     | xiaohei   | changchun |
|     | xiaoli    | dongjing  |
|     | xiaohuang | meiguo    |
|     | xiaoming  | beijing   |
|     | xiaoming  | beijing   |
|     | xiaoming  | beijing   |
+------+-----------+-----------+
 rows in set (0.00 sec)

mysql> alter table user add ziduan char() null;
Query OK,  rows affected (0.05 sec)
Records:   Duplicates:   Warnings: 

mysql> select * from user;
+------+-----------+-----------+--------+
| id   | name      | address   | ziduan |
+------+-----------+-----------+--------+
|     | xiaoming  | beijing   | NULL   |
|     | xiaobai   | shandong  | NULL   |
|     | xiaohong  | suzhou    | NULL   |
|     | xiaohei   | changchun | NULL   |
|     | xiaoli    | dongjing  | NULL   |
|     | xiaohuang | meiguo    | NULL   |
|     | xiaoming  | beijing   | NULL   |
|     | xiaoming  | beijing   | NULL   |
|     | xiaoming  | beijing   | NULL   |
+------+-----------+-----------+--------+
 rows in set (0.00 sec)

mysql> alter table user drop column ziduan;
Query OK,  rows affected (0.06 sec)
Records:   Duplicates:   Warnings: 

mysql> select * from user;
+------+-----------+-----------+
| id   | name      | address   |
+------+-----------+-----------+
|     | xiaoming  | beijing   |
|     | xiaobai   | shandong  |
|     | xiaohong  | suzhou    |
|     | xiaohei   | changchun |
|     | xiaoli    | dongjing  |
|     | xiaohuang | meiguo    |
|     | xiaoming  | beijing   |
|     | xiaoming  | beijing   |
|     | xiaoming  | beijing   |
+------+-----------+-----------+
 rows in set (0.00 sec)