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HomeDatabaseMysql TutorialMySQL学习笔记5:修改表(alter table)

我们在创建表的过程中难免会考虑不周,因此后期会修改表修改表需要用到alter table修改表语句,接下来详细介绍,需要的朋友可以参考下

我们在创建表的过程中难免会考虑不周,因此后期会修改表修改表需要用到alter table语句
修改表名
代码如下:
mysql> alter table student rename person;
Query OK, 0 rows affected (0.03 sec)

这里的student是原名,person是修改过后的名字
用rename来重命名,也可以使用rename to
修改字段的数据类型
代码如下:
mysql> alter table person modify name varchar(20);
Query OK, 0 rows affected (0.18 sec)
Records: 0 Duplicates: 0 Warnings: 0

此处modify后面的name为字段名,我们将原来的varchar(25)改为varchar(20)
修改字段名
代码如下:
mysql> alter table person change stu_name name varchar(25);
Query OK, 0 rows affected (0.20 sec)
Records: 0 Duplicates: 0 Warnings: 0

这里stu_name是原名,name是新名
需要注意的是不管改不改数据类型,后面的数据类型都要写
如果不修改数据类型只需写成原来的数据类型即可
tips:我们同样可以使用change来达到modify的效果,只需在其后写一样的字段名
增加无完整性约束条件的字段
代码如下:
mysql> alter table person add sex boolean;
Query OK, 0 rows affected (0.21 sec)
Records: 0 Duplicates: 0 Warnings: 0

此处的sex后面只跟了数据类型,而没有完整性约束条件
增加有完整性约束条件的字段
代码如下:
mysql> alter table person add age int not null;
Query OK, 0 rows affected (0.17 sec)
Records: 0 Duplicates: 0 Warnings: 0

地处增加了一条age字段,接着在后面加上了not null完整性约束条件
在表头添加字段
代码如下:
mysql> alter table person add num int primary key first;
Query OK, 0 rows affected (0.20 sec)
Records: 0 Duplicates: 0 Warnings: 0

默认情况下添加字段都是添加到表尾,在添加语句后面加上first就能添加到表头
在指定位置添加字段
代码如下:
mysql> alter table person add birth date after name;
Query OK, 0 rows affected (0.20 sec)
Records: 0 Duplicates: 0 Warnings: 0

这里添加一条新字段放在name字段后面
tps:表中字段的排序对表不会有什么影响,不过更合理的排序能便于理解表
删除字段
代码如下:
mysql> alter table person drop sex;
Query OK, 0 rows affected (0.18 sec)
Records: 0 Duplicates: 0 Warnings: 0

和前面删除表或数据库一样,这里也需要用drop
不同的是,删除字段还要用alter table跟着表名
修改字段到第一个位置
代码如下:
mysql> alter table person modify id int first;
Query OK, 0 rows affected (0.20 sec)
Records: 0 Duplicates: 0 Warnings: 0

first在前面已经讲过,此处要注意的是字段后面要写数据类型
修改字段到指定位置
代码如下:
mysql> alter table person modify name varchar(25) after id;
Query OK, 0 rows affected (0.18 sec)
Records: 0 Duplicates: 0 Warnings: 0

我们把name字段放到了id后面,此处的varchar(25)要写全,varchar不行
建议操作以上步骤之前都先desc table
修改表的存储引擎
代码如下:
mysql> alter table user rename person;
Query OK, 0 rows affected (0.05 sec)

这里先不具体讲各个存储引擎的特点,内容比较多
修改完之后别忘了使用show create table语句查看,第三节有写用法
tips:如果表中已存在很多数据,不要轻易修改存储引擎
增加表的外键
代码如下:
mysql> alter table score add constraint fk foreign key(stu_id) references student(id);
Query OK, 10 rows affected (0.18 sec)
Records: 10 Duplicates: 0 Warnings: 0

这里只需使用add增加即可,后面的语法参见第四节中的外键设置
删除表的外键约束
代码如下:
mysql> alter table student3 drop foreign key fk;
Query OK, 0 rows affected (0.18 sec)
Records: 0 Duplicates: 0 Warnings: 0

由于基本的表结构描述无法显示外键,所以在进行此操作前最好使用show create table查看表
这里的fk就是刚刚设置的外键
需要注意的是:如果想要删除有关联的表,那么必先删除外键
删除外键后,原先的key变成普通键
至于删除表的操作,在第三节有写,设置外键在第四节也有写如果创建表的时候没有设置外键,可使用上面的方法
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