gaodaima.com 修改表结构：
alter [ignore] table <table> <table>
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 <em style="color:transparent">本文来源[email protected]搞@^&代*@码)网9</em> | 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: 0mysql> 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 |+-------------+-----------+
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: 4 Duplicates: 0 Warnings: 0+-------------+-----------+| column_name | data_type |+-------------+-----------+| stu_id | int || stu_name | varchar || stu_score | int || tel | int |+-------------+-----------+
3.完整性约束改变
:=add primary keyadd unique add foreign key (column list) referencing add check drop primary keydrop foreign key drop constraint
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);
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