MySQL中的表中增加删除字段

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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

http://blog.csdn.net/qustdjx/article/details/8875827

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