包括表属性改变，列属性改变，完整性约束改变。
1.表属性改变
1.1重命名表名
mysql> alter table student rename to s;
Query OK, 0 rows affected （0.03 sec）
也可以直接使用rename命令改名：
mysql> rename table s to student;
Query OK, 0 rows affected （0.13 sec）
1.2表排序改变
mysql> alter table student order by stu_id desc;
mysql> select * from student;
+--------+----------+---------+-----------+
| stu_id | stu_name | stu_tel | stu_score |
+--------+----------+---------+-----------+
| 4 | d | 154 | 63 |
| 3 | c | 153 | 62 |
| 2 | b | 152 | 61 |
| 1 | a | 151 | 60 |
+--------+----------+---------+-----------+
2列属性改变
2.1 添加列
mysql> alter table student
-> add sex char（1） after stu_name;
Query OK, 4 rows affected （0.34 sec）
Records: 4 Duplicates: 0 Warnings: 0
mysql> select * from student;
+--------+----------+------+---------+-----------+
| stu_id | stu_name | sex | stu_tel | stu_score |
+--------+----------+------+---------+-----------+
| 1 | a | NULL | 151 | 60 |
| 2 | b | NULL | 152 | 61 |
| 3 | c | NULL | 153 | 62 |
| 4 | d | NULL | 154 | 63 |
+--------+----------+------+---------+-----------+
4 rows in set （0.02 sec）
新添加的列默认放在最后一列，且默认填充空值。这里使用after指定了新增列sex放在stu_name后面。如果新增列设置不能为空，那么mysql将根据列的数据类型填入实际的值：对于数值填入0,对于字符串填入空字符串，对于日期填入0000-00-00,对于时间填入00:00:00.
2.2删除列
mysql> alter table student
-> drop sex;
Query OK, 4 rows affected （0.33 sec）
Records: 4 Duplicates: 0 Warnings: 0
2.3修改列属性
初始列属性：
+-------------+-----------+
| column_name | data_type |
+-------------+-----------+
| stu_id | int |
| stu_name | varchar |
| stu_tel | int |
| stu_score | int |
+-------------+-----------+
将stu_tel列修改为tel char型，并放在stu_score后面。
mysql> alter table student
-> change stu_tel tel char（3） after stu_score;;
Query OK, 4 rows affected （0.23 sec）
+-------------+-----------+
| column_name | data_type |
+-------------+-----------+
| stu_id | int |
| stu_name | varchar |
| stu_score | int |
| tel | char |
+-------------+-----------+
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