MySQL_ALTER命令

当我们需要修改数据表名或者修改数据表字段时，就需要使用到MySQL ALTER命令。

1）修改表名：表名可以在数据库中唯一标识一个table

命令格式：ALTER TABLE 旧名 RENAME 新名;

mysql> alter table course rename courses;
Query OK, 0 rows affected (0.00 sec)

2）修改属性的数据类型

命令格式：ALTER TABLE 表名 MODIFY 属性名 数据类型 [完整性约束条件]

mysql> describe num;
+--------+---------+------+-----+---------+----------------+
| Field  | Type    | Null | Key | Default | Extra          |
+--------+---------+------+-----+---------+----------------+
| num_id | int(11) | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10) | NO   | PRI | NULL    |                |
| name   | int(20) | YES  |     | NULL    |                |
+--------+---------+------+-----+---------+----------------+
rows in set (0.00 sec)

mysql> alter table num modify name char(20);  //改变属性的数据类型
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| name   | char(20) | YES  |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
rows in set (0.00 sec)

mysql> alter table num modify sex char(10) not null default 'male'; //改变属性的完整性约束条件
Query OK, 0 rows affected (0.08 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| name    | char(10) | NO   |     | male    |                |
+--------+----------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

3）修改属性的默认值

命令格式1：ALTER TABLE 表名 MODIFY 属性名 数据类型 DEFAULT value;

命令格式2：ALTER TABLE 表名 ALTER 属性名 SET DEFAULT value；

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| sex    | char(10) | NO   |     | male    |                |
+--------+----------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> alter table num modify sex char(10) default 'female';  //修改属性的默认值
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| sex    | char(10) | YES  |     | female  |                |
+--------+----------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

注意：上述示例说明，在对属性做更改使，如果不指定NOT NULL参数，命令会按照“允许为空”的操作进行设置；

而且，若不指定DEFAULT参数，MySQL会自动设置该字段默认为 NULL，即便执行命令之前，其有非NULL默认值。

4）修改字段名

命令格式：ALTER TABLE 表名 CHANGE 属性名 新属性名 新数据类型

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| name   | char(20) | YES  |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
rows in set (0.00 sec)

mysql> alter table num change name sex char(10); //修改属性名称
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| sex    | char(10) | YES  |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
rows in set (0.00 sec)

5）增加字段

命令格式：alter table 表名 ADD 属性名1 [完整性约束条件] [FIRST | AFTER 属性名2]

其中，FIRST参数表示将新加的属性设置为该表的第一个字段；AFTER 属性名2表示将新加的字段置于属性名2（已存在）之后。

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| sex    | char(10) | YES  |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
rows in set (0.00 sec)

mysql> alter table num add name char(20) NOT NULL FIRST;  //新增一个属性
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| name   | char(20) | NO   |     | NULL    |                |
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| sex    | char(10) | YES  |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
rows in set (0.00 sec)

6）删除字段

命令格式：ALTER TABLE 表名 DROP 属性名;

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| name   | char(20) | NO   |     | NULL    |                |
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| sex    | char(10) | YES  |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
rows in set (0.00 sec)

mysql> alter table num drop name;       //删除属性name
Query OK, 0 rows affected (0.09 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> describe num;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| num_id | int(11)  | NO   | PRI | NULL    | auto_increment |
| stu_id | int(10)  | NO   | PRI | NULL    |                |
| sex    | char(10) | YES  |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
rows in set (0.00 sec)