修改表结构：
alter [ignore] table <table specification> <table structure change>
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: 4Duplicates: 0Warnings: 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: 4Duplicates: 0Warnings: 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|
+-------------+-----------+
如果仅仅是更改列其他属性而不改列名时，可以使用modify参数。
将tel列改回int型
mysql> alter table student
-> modify tel int(3);
Query OK, 4 rows affected (0.25 sec)
Records: 4Duplicates: 0Warnings: 0
+-------------+-----------+
| column_name | data_type |
+-------------+-----------+
| stu_id| int|
| stu_name| varchar|
| stu_score| int|
| tel| int|
+-------------+-----------+
3.完整性约束改变
<integrity constraint change>:=
add primary key<index name>
add unique <index name>
add foreign key <index name>(column list) referencing <specification>
add check <condition>
drop primary key
drop foreign key <index name>
drop constraint <constraint name>
在add后面可以添加[constraint ] 为完整性约束定义名字。
删除主键：
mysql> alter table student drop primary key;
添加主键：
mysql> alter table student add primary key(stu_id);
