本文详细介绍了MySQL数据库中对用户表的操作，包括如何增加字段（如new1和new2），删除字段（如new2），修改字段类型和名称，以及进行数据的插入和查询。示例代码清晰展示了SQL语句的使用方法。
1.增加一个字段
alter table user表 add COLUMN new1字段 VARCHAR(20) NOT NULL DEFAULT 0; //增加一个字段,VARCHERA 20 , 不为空,默认值是 0
//增加多个
alter table user表
add COLUMN new1字段 VARCHAR(20) NOT NULL DEFAULT 0,
add COLUMN new2字段 VARCHAR(20) NOT NULL DEFAULT 0;
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
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 | |
+---------+-------------+------+-----+---------+-------+
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 | |
+-------+-------------+------+-----+---------+-------+
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 |
+------+---------+------+---------+
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 |
+------+---------+------+---------+
rows in set (0.00 sec)
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