mysql> select * from user;+------+-----------+-----------+| id   | name      | address   |+------+-----------+-----------+|    1 | xiaoming  | beijing   ||    2 | xiaobai   | shandong  ||    3 | xiaohong  | suzhou    ||    4 | xiaohei   | changchun ||    5 | xiaoli    | dongjing  ||    6 | xiaohuang | meiguo    ||    1 | xiaoming  | beijing   ||    1 | xiaoming  | beijing   ||    1 | xiaoming  | beijing   |+------+-----------+-----------+9 rows in set (0.00 sec)mysql> alter table user add ziduan char(10) null;Query OK, 0 rows affected (0.05 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> select * from user;+------+-----------+-----------+--------+| id   | name      | address   | ziduan |+------+-----------+-----------+--------+|    1 | xiaoming  | beijing   | NULL   ||    2 | xiaobai   | shandong  | NULL   ||    3 | xiaohong  | suzhou    | NULL   ||    4 | xiaohei   | changchun | NULL   ||    5 | xiaoli    | dongjing  | NULL   ||    6 | xiaohuang | meiguo    | NULL   ||    1 | xiaoming  | beijing   | NULL   ||    1 | xiaoming  | beijing   | NULL   ||    1 | xiaoming  | beijing   | NULL   |+------+-----------+-----------+--------+9 rows in set (0.00 sec)mysql> alter table user drop column ziduan;Query OK, 0 rows affected (0.06 sec)Records: 0  Duplicates: 0  Warnings: 0mysql> select * from user;+------+-----------+-----------+| id   | name      | address   |+------+-----------+-----------+|    1 | xiaoming  | beijing   ||    2 | xiaobai   | shandong  ||    3 | xiaohong  | suzhou    ||    4 | xiaohei   | changchun ||    5 | xiaoli    | dongjing  ||    6 | xiaohuang | meiguo    ||    1 | xiaoming  | beijing   ||    1 | xiaoming  | beijing   ||    1 | xiaoming  | beijing   |+------+-----------+-----------+9 rows in set (0.00 sec)