MySQL中的表中增加删除字段

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: 0

 mysql> 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: 0

 mysql> 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: 0

 mysql> 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