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