基本上每种数据库修改表结构都是使用alter table语句，MySQL也不例外，其通过alter table语句的add、change、modify等不同的选项可以实现字段增加和修改。
例如，增加字段

root@database-one 22:53:  [gftest]> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field    | Type          | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename    | varchar(10)   | YES  |     | NULL    |       |
| hiredate | date          | YES  |     | NULL    |       |
| sal      | decimal(10,2) | YES  |     | NULL    |       |
| deptno   | int(2)        | YES  |     | NULL    |       |
+----------+---------------+------+-----+---------+-------+
4 rows in set (0.33 sec)

root@database-one 22:53:  [gftest]> alter table emp add column age int;
Query OK, 0 rows affected (0.91 sec)
Records: 0  Duplicates: 0  Warnings: 0

root@database-one 22:54:  [gftest]> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field    | Type          | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename    | varchar(10)   | YES  |     | NULL    |       |
| hiredate | date          | YES  |     | NULL    |       |
| sal      | decimal(10,2) | YES  |     | NULL    |       |
| deptno   | int(2)        | YES  |     | NULL    |       |
| age      | int(11)       | YES  |     | NULL    |       |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

常规的增加字段，数据库都是将新增的字段放到最后面，但是MySQL中却有个有意思的小特性，可以在增加字段时指定字段在表中的位置。比如我们想将新增的age字段放到ename和hiredate之间

root@database-one 22:54:  [gftest]> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field    | Type          | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename    | varchar(10)   | YES  |     | NULL    |       |
| hiredate | date          | YES  |     | NULL    |       |
| sal      | decimal(10,2) | YES  |     | NULL    |       |
| deptno   | int(2)        | YES  |     | NULL    |       |
| age      | int(11)       | YES  |     | NULL    |       |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

root@database-one 22:54:  [gftest]> 
root@database-one 22:58:  [gftest]> 
root@database-one 22:58:  [gftest]> alter table emp drop column age;
Query OK, 0 rows affected (0.34 sec)
Records: 0  Duplicates: 0  Warnings: 0

root@database-one 23:00:  [gftest]> alter table emp add column age int after ename;
Query OK, 0 rows affected (0.13 sec)
Records: 0  Duplicates: 0  Warnings: 0

root@database-one 23:00:  [gftest]> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field    | Type          | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename    | varchar(10)   | YES  |     | NULL    |       |
| age      | int(11)       | YES  |     | NULL    |       |
| hiredate | date          | YES  |     | NULL    |       |
| sal      | decimal(10,2) | YES  |     | NULL    |       |
| deptno   | int(2)        | YES  |     | NULL    |       |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.06 sec)

也可以修改已存在字段的位置，比如将hiredate放到sal后边

root@database-one 23:00:  [gftest]> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field    | Type          | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename    | varchar(10)   | YES  |     | NULL    |       |
| age      | int(11)       | YES  |     | NULL    |       |
| hiredate | date          | YES  |     | NULL    |       |
| sal      | decimal(10,2) | YES  |     | NULL    |       |
| deptno   | int(2)        | YES  |     | NULL    |       |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.06 sec)

root@database-one 23:05:  [gftest]> 
root@database-one 23:05:  [gftest]> alter table emp modify hiredate date after sal;
Query OK, 0 rows affected (0.57 sec)
Records: 0  Duplicates: 0  Warnings: 0

root@database-one 23:05:  [gftest]> desc emp;
+----------+---------------+------+-----+---------+-------+
| Field    | Type          | Null | Key | Default | Extra |
+----------+---------------+------+-----+---------+-------+
| ename    | varchar(10)   | YES  |     | NULL    |       |
| age      | int(11)       | YES  |     | NULL    |       |
| sal      | decimal(10,2) | YES  |     | NULL    |       |
| hiredate | date          | YES  |     | NULL    |       |
| deptno   | int(2)        | YES  |     | NULL    |       |
+----------+---------------+------+-----+---------+-------+
5 rows in set (0.06 sec)