mysql 增加或删除表里的字段

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

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1增加两个字段：

[sql] view plain copy

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)