假设我们存在user表，user结构如下：
> mysql> desc user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| username | varchar(10) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.01 sec)
## 修改表字段类型 modify
| 类别 | 详细解示 |
| -- | -- |
| 基本语法 | alter table 表名 modify 字段名 varchar(20); |
| 示例 |alter table user modify username varchar(20); |
| 示例说明 | 将user表的username的类型改为varchar(20) |
我们执行一下，看看结果：
> mysql> alter table user modify username varchar(20);
Query OK, 0 rows affected (0.48 sec)
Records: 0 Duplicates: 0 Warnings: 0
> mysql> desc user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| username | varchar(20) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.01 sec)
##增加表字段
| 类别 | 详细解示 |
| -- | -- |
| 基本语法 | alter table 表名 add column 字段名 类型; |
| 示例 |alter table user add column age int(3); |
| 示例说明 | 添加一个字段为age，类型为整型长度为3 |
> mysql> alter table emp add column age int(3);
Query OK, 0 rows affected (0.40 sec)
Records: 0 Duplicates: 0 Warnings: 0
> mysql> desc user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| username | varchar(20) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
| age | int(3) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
##增加字段时控制字段顺序
我们刚刚学了增加字段。如果你仔细实验发现每次都是增加在最后面，如何在第一个增加或者在指字字段之后增加呢？
| 类别 | 详细解释 |
| -- | -- |
| 基本语法 | ALTER TABLE 表名 ADD 字段名 字段类型 AFTER 字段名; |
| 示例 |ALTER TABLE user ADD email VARCHAR(60) AFTER createip; |
| 示例说明 | user表中，在createip后增加一个字段为email，类型为varchar，长度为60 |
| 类别 | 详细解示 |
| -- | -- |
| 基本语法 | ALTER TABLE 表名 ADD 字段名 字段类型;|
| 示例 |ALTER TABLE user ADD id INT(10) FIRST; |
| 示例说明 | user表中在最开始的位置增加一个字段为id,类型为int，长度为10 |
> ALTER TABLE user ADD email VARCHAR(60) AFTER createip;
Query OK, 0 rows affected (0.40 sec)
Records: 0 Duplicates: 0 Warnings: 0
> mysql> desc user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| username | varchar(20) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
| email | varchar(60) | YES | | NULL | |
| age | int(3) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
6 rows in set (0.00 sec)
##删除表字段
| 类别 | 详细解示 |
| -- | -- |
| 基本语法 | alter table 表名 drop column 字段名; |
| 示例 | alter table user drop column age; |
| 示例说明 | 在user表中删除字段age |
> mysql> alter table user drop column age;
Query OK, 0 rows affected (0.27 sec)
Records: 0 Duplicates: 0 Warnings: 0
> mysql> desc user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| username | varchar(20) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
| email | varchar(60) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
##表字段改名
| 类别 | 详细解示 |
| -- | -- |
| 基本语法 | alter table 表名 change 字段原名 字段新名 字段类型; |
| 示例 | alter table user change email em varchar(60); |
| 示例说明 | 在user表中将字段中的email字段名字为em |
详细示例：
mysql> alter table user change email em varchar(60);
Query OK, 0 rows affected (0.38 sec)
Records: 0 Duplicates: 0 Warnings: 0
> mysql> desc user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| username | varchar(20) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
| em | varchar(60) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
##修改表字段排列顺序
在前的字段增加和修改语句（add/change/modify）中，最后都可以加一个可选项 first|after。
增加表字段时我们已经学过了如何调整顺序。我们现在在来看看另外的change或modify如何来调整顺序。
我们用first做个小实验。
###使用modify调整顺序
> mysql> alter table user modify em varchar(60) first;
Query OK, 0 rows affected (0.41 sec)
Records: 0 Duplicates: 0 Warnings: 0
> mysql> desc user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| em | varchar(60) | YES | | NULL | |
| username | varchar(20) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
##修改表名
| 类别 | 详细解示 |
| -- | -- |
| 基本语法 | alter table 旧表名 rename 新的表名; |
| 示例 | alter table user rename new_user; |
| 示例说明 | 将user表名改为new_user |
> mysql> alter table user rename new_user;
Query OK, 0 rows affected (0.35 sec)
> mysql> desc new_user;
+----------+---------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| em | varchar(60) | YES | | NULL | |
| username | varchar(20) | YES | | NULL | |
| password | varchar(32) | YES | | NULL | |
| createtime | int(10) | YES | | NULL | |
| createip | int(10) | YES | | NULL | |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)
