6：列属性完整性 -MySQL
作者：Yeats_Liao
- 2022-10-26 江西
本文字数：8708 字
阅读完需：约 29 分钟
6.1 列属性问题
列属性要根据业务的要求来对数据的一些控制，例如是否能为空，是否是唯一的，等各种操作，就是我们每次desc表的时候的表头上的内容就是与类属性有关的东西：Type Null Key Dafult
6.2 Primary key 主键作用以及企业用途
主键作用：用来区分数据，用来联系各表
特点就是：不能为空且在表中的数据是唯一的
不能为空保证了数据的完整性，在表中唯一使得在数据查询中变得更加方便，加快了表的查询速度
主键的设计非常重要，那么如何定义主键呢？
注意：在非自增字段的主键，
Default必须赋值，不能为NULL
mysql> create table t_8( -> id int(20) primary key, -> name varchar(30) -> );Query OK, 0 rows affected (0.02 sec)
mysql> desc t_8;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(20) | NO | PRI | NULL | || name | varchar(30) | YES | | NULL | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)
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将某数据修改成主键的方法
alter table 表名 add primary key (字段名)；
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mysql> alter table pet_user add primary key (id);Query OK, 0 rows affected (0.04 sec)Records: 0 Duplicates: 0 Warnings: 0
mysql> desc pet_user;+-------+---------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| id | int(10) | NO | PRI | NULL | || age | int(9) | YES | | NULL | |+-------+---------+------+-----+---------+-------+2 rows in set (0.00 sec)
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6.3 删除主键、组合键、选择主键
1. 删除主键
alter table 表名 drop primary key；
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mysql> alter table t_8 drop primary key;Query OK, 0 rows affected (0.04 sec)Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t_8;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(20) | NO | | NULL | || name | varchar(30) | YES | | NULL | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)
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2. 组合键
可以添加组合键（复合主键），但扩展性确实不行
一张表里可以有多个组合键，但是可能在非特殊的情况下实际的意义不大
添加复合主键要先把原主键删除
alter table 表名 add primary key （字段，字段...）;
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mysql> alter table t_8 add primary key (id,name);Query OK, 0 rows affected (0.03 sec)Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t_8;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(20) | NO | PRI | NULL | || name | varchar(30) | NO | PRI | NULL | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)
mysql>
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3. 选择主键
主键的选择很重要，选择有把握且绝对不会更改的数据，最好是数字
万一所给你的数据没有这种类型的数据的时候，我们需要自己创建‘id’
6.4 复合主键究竟有什么用？
组和键运用范围？
例如用 B 站和微博，我们的 id 是唯一的，昵称也是唯一的，
这个时候可以参考组合键的方式，但现在使用的是唯一键，
注意：组合键并不是一个表中有多个主键，而是这些字段组合成复合主键，所以主键在表中只能有一个
6.5 unique 唯一键的作用以及使用
唯一键限定范围是在一张表中，它不会用作来关联其他的数据
它也可以为空，在这张表中一定是唯一的，用来保证在这个表中这个数据不重复
和添加主键一样，有两种方法：
在创建表的时候数据写上unique
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mysql> create table t_9( -> id int primary key, -> phone varchar(20) unique -> );Query OK, 0 rows affected (0.02 sec)
mysql> desc t_9;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(11) | NO | PRI | NULL | || phone | varchar(20) | YES | UNI | NULL | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)
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alter table 表名 add unique （字段，字段...）;
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mysql> create table t_10( -> id int(4), -> phone varchar(20) -> );Query OK, 0 rows affected (0.02 sec)
mysql> desc t_10;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(4) | YES | | NULL | || phone | varchar(20) | YES | | NULL | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)
mysql> alter table t_10 add unique(phone);Query OK, 0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t_10;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(4) | YES | | NULL | || phone | varchar(20) | YES | UNI | NULL | |+-------+-------------+------+-----+---------+-------+2 rows in set (0.00 sec)
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6.6 唯一键扩展
删除唯一键
alter table 表名 drop index 字段名 字段名不能使用括号
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mysql> create table t_11( -> id int, -> name varchar(20) unique, -> phone varchar(20) unique -> );Query OK, 0 rows affected (0.06 sec)
mysql> desc t_11;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(11) | YES | | NULL | || name | varchar(20) | YES | UNI | NULL | || phone | varchar(20) | YES | UNI | NULL | |+-------+-------------+------+-----+---------+-------+3 rows in set (0.00 sec)
mysql> alter table t_11 drop index phone;Query OK, 0 rows affected (0.02 sec)Records: 0 Duplicates: 0 Warnings: 0
mysql> desc t_11;+-------+-------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+-------------+------+-----+---------+-------+| id | int(11) | YES | | NULL | || name | varchar(20) | YES | UNI | NULL | || phone | varchar(20) | YES | | NULL | |+-------+-------------+------+-----+---------+-------+3 rows in set (0.00 sec)
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6.7 主键和唯一键区别
6.8 sql 内注释代码注释
1. 普通注释
命令单行注释与多行注释
mysql> create table t_12 ( -> id int(20) primary key,# this is primary key! -> phone varchar(20) -- this is phone -> /* /*> abcdefg /*> hijklmn /*> */ -> );Query OK, 0 rows affected (0.03 sec)
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2. sql 内注释
就是 comment 注释
mysql> create table staff( -> id int auto_increment primary key comment'主键id', -> name varchar(30) not null, -> age int comment'年龄', -> salary int default '0' comment'薪水' -> )engine=innodb; mysql> show create table staff;+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table |+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| staff | CREATE TABLE `staff` ( `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键id', `name` varchar(30) NOT NULL, `age` int(11) DEFAULT NULL COMMENT '年龄', `salary` int(11) DEFAULT '0' COMMENT '薪水', PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=gbk |+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)
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6.9 数据库完整性
考虑数据库的完整性
一个表一定要有一个主键来约束
保证数据类型一定要是对的
哪些字段可以为空哪些不行
Defalut的使用可能需要对外部的引用
6.10 引用数据表的完整性问题，抛出外键的概念
外部引用问题，如何关联两个表，将公共的字段链接在一起呢？
我们创建表时一定要明确主表和从表
6.11 外键
创建外键
foreign key (数据) references 连接的表名(数据));
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mysql> create table eatery( -> id int primary key, -> money decimal(10,4), -> stuId int(4), -> foreign key (stuId) references stu(stuId));Query OK, 0 rows affected (0.03 sec)
mysql> desc eatery;+-------+---------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+---------------+------+-----+---------+-------+| id | int(11) | NO | PRI | NULL | || money | decimal(10,4) | YES | | NULL | || stuId | int(4) | YES | MUL | NULL | |+-------+---------------+------+-----+---------+-------+3 rows in set (0.00 sec)
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alter table 表名 add foreign key (数据) references 连接的表名(数据);
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mysql> create table eatery_2( -> id int primary key, -> money decimal(10,4), -> stuId int (4) -> );Query OK, 0 rows affected (0.03 sec)
mysql> desc eatery_2;+-------+---------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+---------------+------+-----+---------+-------+| id | int(11) | NO | PRI | NULL | || money | decimal(10,4) | YES | | NULL | || stuId | int(4) | YES | | NULL | |+-------+---------------+------+-----+---------+-------+3 rows in set (0.00 sec)
mysql> alter table eatery_2 add foreign key (stuId) references stu(stuId);Query OK, 0 rows affected (0.04 sec)Records: 0 Duplicates: 0 Warnings: 0
mysql> desc eatery_2;+-------+---------------+------+-----+---------+-------+| Field | Type | Null | Key | Default | Extra |+-------+---------------+------+-----+---------+-------+| id | int(11) | NO | PRI | NULL | || money | decimal(10,4) | YES | | NULL | || stuId | int(4) | YES | MUL | NULL | |+-------+---------------+------+-----+---------+-------+3 rows in set (0.00 sec)
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6.12 什么时候设计外键呢？
在设计表初时就要考虑到外键，一旦创建很少修改，后期维护可能会修改
6.13 更正上节课错误，删除外键
先要找到这个外键的名字再删除，查看外键名
show create table 表名
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找到外键名在CONSTRAINT后
CONSTRAINT `xxxxxx` FOREIGN KEY (`xxxx`) REFERENCES `xxx (`xx`)
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删除外键
alter table 表名 drop foreign key 外键名
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mysql> show create table eatery;+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table
|+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| eatery | CREATE TABLE `eatery` ( `id` int(11) NOT NULL, `money` decimal(10,4) DEFAULT NULL, `stuId` int(4) DEFAULT NULL, PRIMARY KEY (`id`), KEY `stuId` (`stuId`), CONSTRAINT `eatery_ibfk_1` FOREIGN KEY (`stuId`) REFERENCES `stu` (`stuId`)) ENGINE=InnoDB DEFAULT CHARSET=gbk |+--------+----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)
mysql> alter table eatery drop foreign key eatery_ibfk_1;Query OK, 0 rows affected (0.01 sec)Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table eatery;+--------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table
|+--------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| eatery | CREATE TABLE `eatery` ( `id` int(11) NOT NULL, `money` decimal(10,4) DEFAULT NULL, `stuId` int(4) DEFAULT NULL, PRIMARY KEY (`id`), KEY `stuId` (`stuId`)) ENGINE=InnoDB DEFAULT CHARSET=gbk |+--------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)
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6.14 外键三种操作：严格、置空、级联的使用场景以及介绍
1. 严格性操作
也就是增删改查表的规范
2. 置空操作
链接外键的表中，原表删除的数据会成为 NULL
3. 级联操作
链接外键的表中，原表删除的数据会全部删除
注意：留给外键进行删除数据的时候使用置空，更新使用级联
6.15 置空和级联演示
创建eatery表，stuId为外键，设置级联和置空操作
foreign key(数据) references 表名(数据) on delete set null on update cascade
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mysql> create table stu( -> stuId int (4) primary key, -> name varchar(20) -> );Query OK, 0 rows affected (0.02 sec)
mysql> create table eatery( -> id int(20) primary key, -> money decimal(10,4), -> stuId int(4), -> foreign key(stuId) references stu(stuId) on delete set null on update cascade -> );Query OK, 0 rows affected (0.02 sec)
mysql> show create table eatery;+--------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| Table | Create Table
|+--------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+| eatery | CREATE TABLE `eatery` ( `id` int(20) NOT NULL, `money` decimal(10,4) DEFAULT NULL, `stuId` int(4) DEFAULT NULL, PRIMARY KEY (`id`), KEY `stuId` (`stuId`), CONSTRAINT `eatery_ibfk_1` FOREIGN KEY (`stuId`) REFERENCES `stu` (`stuId`) ON DELETE SET NULL ON UPDATE CASCADE) ENGINE=InnoDB DEFAULT CHARSET=gbk |+--------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+1 row in set (0.00 sec)
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插入数据
mysql> insert into stu values(1,'frank');Query OK, 1 row affected (0.01 sec)
mysql> insert into stu values(2,'jerry');Query OK, 1 row affected (0.00 sec)
mysql> select * from stu;+-------+-------+| stuId | name |+-------+-------+| 1 | frank || 2 | jerry |+-------+-------+2 rows in set (0.00 sec)
mysql> insert into eatery values(1, 20.5, 2);Query OK, 1 row affected (0.00 sec)
mysql> select * from eatery;+----+---------+-------+| id | money | stuId |+----+---------+-------+| 1 | 20.5000 | 2 |+----+---------+-------+1 row in set (0.00 sec)
mysql> insert into eatery values(2, 78.6, 1);Query OK, 1 row affected (0.01 sec)
mysql> insert into eatery values(3, 99.9, 2);Query OK, 1 row affected (0.01 sec)
mysql> insert into eatery values(4, 748.4, 1);Query OK, 1 row affected (0.00 sec)
mysql> insert into eatery values(5, 748.4, 2);Query OK, 1 row affected (0.01 sec)
mysql> select * from eatery;+----+----------+-------+| id | money | stuId |+----+----------+-------+| 1 | 20.5000 | 2 || 2 | 78.6000 | 1 || 3 | 99.9000 | 2 || 4 | 748.4000 | 1 || 5 | 748.4000 | 2 |+----+----------+-------+5 rows in set (0.00 sec)
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更新stu表中stuId时，eatery表外键的数据会相应更改，这里是级联操作
mysql> update stu set stuId='4' where name = 'frank';Query OK, 1 row affected (0.01 sec)Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from eatery;+----+----------+-------+| id | money | stuId |+----+----------+-------+| 1 | 20.5000 | 2 || 2 | 78.6000 | 4 || 3 | 99.9000 | 2 || 4 | 748.4000 | 4 || 5 | 748.4000 | 2 |+----+----------+-------+5 rows in set (0.00 sec)
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删除stu表中stuId时，eatery表外键的数据会变为NULL，这里是置空操作
mysql> delete from stu where stuId='2';Query OK, 1 row affected (0.01 sec)
mysql> select * from eatery;+----+----------+-------+| id | money | stuId |+----+----------+-------+| 1 | 20.5000 | NULL || 2 | 78.6000 | 4 || 3 | 99.9000 | NULL || 4 | 748.4000 | 4 || 5 | 748.4000 | NULL |+----+----------+-------+5 rows in set (0.00 sec)
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