1增加两个字段：
mysql> create table id_name(id int,name varchar(20)); Query OK, 0 rows affected (0.13 sec)mysql> alter table id_name add age int,add address varchar(11); Query OK, 0 rows affected (0.13 sec) Records: 0 Duplicates: 0 Warnings: 0mysql> desc id_name；+---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | | age | int(11) | YES | | NULL | | | address | varchar(11) | YES | | NULL | | +---------+-------------+------+-----+---------+-------+ 4 rows in set (0.00 sec)2.删除两个字段 mysql> alter table id_name drop column age,drop column address; Query OK, 0 rows affected (0.14 sec) Records: 0 Duplicates: 0 Warnings: 0mysql> desc id_name; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | id | int(11) | YES | | NULL | | | name | varchar(20) | YES | | NULL | | +-------+-------------+------+-----+---------+-------+ 2 rows in set (0.00 sec)3.插入 mysql> insert into id_name values (1,'qustdjx'); Query OK, 1 row affected (0.00 sec) 4.查询看一下 mysql> alter table id_name add age int,add address varchar(11); Query OK, 1 row affected (0.07 sec) Records: 1 Duplicates: 0 Warnings: 0mysql> select * from id_name; +------+---------+------+---------+ | id | name | age | address | +------+---------+------+---------+ | 1 | qustdjx | NULL | NULL | +------+---------+------+---------+ 1 row in set (0.00 sec) 5.新增字段并插入 mysql> insert into id_name values(2,'qust',14,'山东'); Query OK, 1 row affected (0.00 sec)mysql> select * from id_name; +------+---------+------+---------+ | id | name | age | address | +------+---------+------+---------+ | 1 | qustdjx | NULL | NULL | | 2 | qust | 14 | 山东 | +------+---------+------+---------+ 2 rows in set (0.00 sec)
1.增加一个字段
alter table user add COLUMN new1 VARCHAR(20) DEFAULT NULL; //增加一个字段，默认为空
alter table user add COLUMN new2 VARCHAR(20) NOT NULL; //增加一个字段，默认不能为空 www.2cto.com
2.删除一个字段
alter table user DROP COLUMN new2; //删除一个字段
3.修改一个字段
alter table user MODIFY new1 VARCHAR(10); //修改一个字段的类型
alter table user CHANGE new1 new4 int; //修改一个字段的名称，此时一定要重新指定该字段的类型
