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  • 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)
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  • 原文地址:https://www.cnblogs.com/yy20141204bb/p/8409504.html
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