MySQL四-2:完整性约束

阅读目录

• 一 介绍
• 二 not null与default
• 三 unique
• 四 primary key
• 五 auto_increment
• 六 foreign key
• 七 作业

一 介绍

约束条件与数据类型的宽度一样，都是可选参数

作用：用于保证数据的完整性和一致性
主要分为：

PRIMARY KEY (PK)    标识该字段为该表的主键，可以唯一的标识记录
FOREIGN KEY (FK)    标识该字段为该表的外键
NOT NULL    标识该字段不能为空
UNIQUE KEY (UK)    标识该字段的值是唯一的
AUTO_INCREMENT    标识该字段的值自动增长（整数类型，而且为主键）
DEFAULT    为该字段设置默认值
 
UNSIGNED 无符号
ZEROFILL 使用0填充

说明：

1. 是否允许为空，默认NULL，可设置NOT NULL，字段不允许为空，必须赋值
2. 字段是否有默认值，缺省的默认值是NULL，如果插入记录时不给字段赋值，此字段使用默认值
sex enum('male','female') not null default 'male'
age int unsigned NOT NULL default 20 必须为正值（无符号） 不允许为空 默认是20
3. 是否是key
主键 primary key
外键 foreign key
索引 (index,unique...)

二 not null与default

是否可空，null表示空，非字符串
not null - 不可空
null - 可空

默认值，创建列时可以指定默认值，当插入数据时如果未主动设置，则自动添加默认值
create table tb1(
nid int not null defalut 2,
num int not null)

==================not null====================
mysql> create table t1(id int); #id字段默认可以插入空
mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | YES  |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
mysql> insert into t1 values(); #可以插入空
 
mysql> create table t2(id int not null); #设置字段id不为空
mysql> desc t2;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
mysql> insert into t2 values(); #不能插入空
ERROR 1364 (HY000): Field 'id' doesn't have a default value
 
==================default====================
#设置id字段有默认值后，则无论id字段是null还是not null，都可以插入空，插入空默认填入default指定的默认值
mysql> create table t3(id int default 1);
mysql> alter table t3 modify id int not null default 1;
 
==================综合练习====================
mysql> create table student(
    -> name varchar(20) not null,
    -> age int(3) unsigned not null default 18,
    -> sex enum('male','female') default 'male',
    -> hobby set('play','study','read','music') default 'play,music'
    -> );
mysql> desc student;
+-------+------------------------------------+------+-----+------------+-------+
| Field | Type                               | Null | Key | Default    | Extra |
+-------+------------------------------------+------+-----+------------+-------+
| name  | varchar(20)                        | NO   |     | NULL       |       |
| age   | int(3) unsigned                    | NO   |     | 18         |       |
| sex   | enum('male','female')              | YES  |     | male       |       |
| hobby | set('play','study','read','music') | YES  |     | play,music |       |
+-------+------------------------------------+------+-----+------------+-------+
mysql> insert into student(name) values('egon');
mysql> select * from student;
+------+-----+------+------------+
| name | age | sex  | hobby      |
+------+-----+------+------------+
| egon |  18 | male | play,music |
+------+-----+------+------------+

验证

三 unique

============设置唯一约束 UNIQUE===============
方法一：
create table department1(
id int,
name varchar(20) unique,
comment varchar(100)
);
 
方法二：
create table department2(
id int,
name varchar(20),
comment varchar(100),
constraint uk_name unique(name)
);
 
mysql> insert into department1 values(1,'IT','技术');
Query OK, 1 row affected (0.00 sec)
mysql> insert into department1 values(1,'IT','技术');
ERROR 1062 (23000): Duplicate entry 'IT' for key 'name'

mysql> create table t1(id int not null unique);
Query OK, 0 rows affected (0.02 sec)
 
mysql> desc t1;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   | PRI | NULL    |       |
+-------+---------+------+-----+---------+-------+
row in set (0.00 sec)

not null+unique的化学反应

create table service(
id int primary key auto_increment,
name varchar(20),
host varchar(15) not null,
port int not null,
unique(host,port) #联合唯一
);
 
mysql> insert into service values
    -> (1,'nginx','192.168.0.10',80),
    -> (2,'haproxy','192.168.0.20',80),
    -> (3,'mysql','192.168.0.30',3306)
    -> ;
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0
 
mysql> insert into service(name,host,port) values('nginx','192.168.0.10',80);
ERROR 1062 (23000): Duplicate entry '192.168.0.10-80' for key 'host'

联合唯一

四 primary key

primary key字段的值不为空且唯一

一个表中可以：

单列做主键
多列做主键（复合主键）

但一个表内只能有一个主键primary key

============单列做主键===============
#方法一：not null+unique
create table department1(
id int not null unique, #主键
name varchar(20) not null unique,
comment varchar(100)
);
 
mysql> desc department1;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | NO   | UNI | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)
 
#方法二：在某一个字段后用primary key
create table department2(
id int primary key, #主键
name varchar(20),
comment varchar(100)
);
 
mysql> desc department2;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.00 sec)
 
#方法三：在所有字段后单独定义primary key
create table department3(
id int,
name varchar(20),
comment varchar(100),
constraint pk_name primary key(id); #创建主键并为其命名pk_name
 
mysql> desc department3;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| id      | int(11)      | NO   | PRI | NULL    |       |
| name    | varchar(20)  | YES  |     | NULL    |       |
| comment | varchar(100) | YES  |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
rows in set (0.01 sec)

单列主键

==================多列做主键================
create table service(
ip varchar(15),
port char(5),
service_name varchar(10) not null,
primary key(ip,port)
);
 
mysql> desc service;
+--------------+-------------+------+-----+---------+-------+
| Field        | Type        | Null | Key | Default | Extra |
+--------------+-------------+------+-----+---------+-------+
| ip           | varchar(15) | NO   | PRI | NULL    |       |
| port         | char(5)     | NO   | PRI | NULL    |       |
| service_name | varchar(10) | NO   |     | NULL    |       |
+--------------+-------------+------+-----+---------+-------+
rows in set (0.00 sec)
 
mysql> insert into service values
    -> ('172.16.45.10','','mysqld'),
    -> ('172.16.45.11','','mariadb')
    -> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0
 
mysql> insert into service values ('172.16.45.10','','nginx');
ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'

多列主键

五 auto_increment

约束字段为自动增长，被约束的字段必须同时被key约束

#不指定id，则自动增长
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
);
 
mysql> desc student;
+-------+-----------------------+------+-----+---------+----------------+
| Field | Type                  | Null | Key | Default | Extra          |
+-------+-----------------------+------+-----+---------+----------------+
| id    | int(11)               | NO   | PRI | NULL    | auto_increment |
| name  | varchar(20)           | YES  |     | NULL    |                |
| sex   | enum('male','female') | YES  |     | male    |                |
+-------+-----------------------+------+-----+---------+----------------+
mysql> insert into student(name) values
    -> ('egon'),
    -> ('alex')
    -> ;
 
mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | egon | male |
|  2 | alex | male |
+----+------+------+
 
#也可以指定id
mysql> insert into student values(4,'asb','female');
Query OK, 1 row affected (0.00 sec)
 
mysql> insert into student values(7,'wsb','female');
Query OK, 1 row affected (0.00 sec)
 
mysql> select * from student;
+----+------+--------+
| id | name | sex    |
+----+------+--------+
|  1 | egon | male   |
|  2 | alex | male   |
|  4 | asb  | female |
|  7 | wsb  | female |
+----+------+--------+
 
#对于自增的字段，在用delete删除后，再插入值，该字段仍按照删除前的位置继续增长
mysql> delete from student;
Query OK, 4 rows affected (0.00 sec)
 
mysql> select * from student;
Empty set (0.00 sec)
 
mysql> insert into student(name) values('ysb');
mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  8 | ysb  | male |
+----+------+------+
 
#应该用truncate清空表，比起delete一条一条地删除记录，truncate是直接清空表，在删除大表时用它
mysql> truncate student;
Query OK, 0 rows affected (0.01 sec)
 
mysql> insert into student(name) values('egon');
Query OK, 1 row affected (0.01 sec)
 
mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  1 | egon | male |
+----+------+------+
row in set (0.00 sec)

#在创建完表后，修改自增字段的起始值
mysql> create table student(
    -> id int primary key auto_increment,
    -> name varchar(20),
    -> sex enum('male','female') default 'male'
    -> );
 
mysql> alter table student auto_increment=3;
 
mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8
 
mysql> insert into student(name) values('egon');
Query OK, 1 row affected (0.01 sec)
 
mysql> select * from student;
+----+------+------+
| id | name | sex  |
+----+------+------+
|  3 | egon | male |
+----+------+------+
row in set (0.00 sec)
 
mysql> show create table student;
.......
ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8
 
#也可以创建表时指定auto_increment的初始值，注意初始值的设置为表选项，应该放到括号外
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
)auto_increment=3;
 
#设置步长
sqlserver：自增步长
    基于表级别
    create table t1（
        id int。。。
    ）engine=innodb,auto_increment=2 步长=2 default charset=utf8
 
mysql自增的步长：
    show session variables like 'auto_inc%';
 
    #基于会话级别
    set session auth_increment_increment=2 #修改会话级别的步长
 
    #基于全局级别的
    set global auth_increment_increment=2 #修改全局级别的步长（所有会话都生效）
 
#！！！注意了注意了注意了！！！
If the value of auto_increment_offset is greater than that of auto_increment_increment, the value of auto_increment_offset is ignored.
翻译：如果auto_increment_offset的值大于auto_increment_increment的值，则auto_increment_offset的值会被忽略
比如：设置auto_increment_offset=3，auto_increment_increment=2
 
mysql> set global auto_increment_increment=5;
Query OK, 0 rows affected (0.00 sec)
 
mysql> set global auto_increment_offset=3;
Query OK, 0 rows affected (0.00 sec)
 
mysql> show variables like 'auto_incre%'; #需要退出重新登录
+--------------------------+-------+
| Variable_name            | Value |
+--------------------------+-------+
| auto_increment_increment | 1     |
| auto_increment_offset    | 1     |
+--------------------------+-------+
 
create table student(
id int primary key auto_increment,
name varchar(20),
sex enum('male','female') default 'male'
);
 
mysql> insert into student(name) values('egon1'),('egon2'),('egon3');
mysql> select * from student;
+----+-------+------+
| id | name  | sex  |
+----+-------+------+
|  3 | egon1 | male |
|  8 | egon2 | male |
| 13 | egon3 | male |
+----+-------+------+

步长increment与起始偏移量offset:auto_increment_increment,auto_increment_offset

六 foreign key

员工信息表有三个字段：工号  姓名  部门

公司有3个部门，但是有1个亿的员工，那意味着部门这个字段需要重复存储，部门名字越长，越浪费

解决方法：

我们完全可以定义一个部门表

然后让员工信息表关联该表，如何关联，即foreign key

#表类型必须是innodb存储引擎，且被关联的字段，即references指定的另外一个表的字段，必须保证唯一
create table department(
id int primary key,
name varchar(20) not null
)engine=innodb;
 
#dpt_id外键，关联父表（department主键id），同步更新，同步删除
create table employee(
id int primary key,
name varchar(20) not null,
dpt_id int,
constraint fk_name foreign key(dpt_id)
references department(id)
on delete cascade
on update cascade
)engine=innodb;
 
#先往父表department中插入记录
insert into department values
(1,'欧德博爱技术有限事业部'),
(2,'艾利克斯人力资源部'),
(3,'销售部');
 
#再往子表employee中插入记录
insert into employee values
(1,'egon',1),
(2,'alex1',2),
(3,'alex2',2),
(4,'alex3',2),
(5,'李坦克',3),
(6,'刘飞机',3),
(7,'张火箭',3),
(8,'林子弹',3),
(9,'加特林',3)
;
 
#删父表department，子表employee中对应的记录跟着删
mysql> delete from department where id=3;
mysql> select * from employee;
+----+-------+--------+
| id | name  | dpt_id |
+----+-------+--------+
|  1 | egon  |      1 |
|  2 | alex1 |      2 |
|  3 | alex2 |      2 |
|  4 | alex3 |      2 |
+----+-------+--------+
 
#更新父表department，子表employee中对应的记录跟着改
mysql> update department set id=22222 where id=2;
mysql> select * from employee;
+----+-------+--------+
| id | name  | dpt_id |
+----+-------+--------+
|  1 | egon  |      1 |
|  3 | alex2 |  22222 |
|  4 | alex3 |  22222 |
|  5 | alex1 |  22222 |
+----+-------+--------+

示范

表1 foreign key 表2
则表1的多条记录对应表2的一条记录，即多对一
 
利用foreign key的原理我们可以制作两张表的多对多，一对一关系
多对多：
    表1的多条记录可以对应表2的一条记录
    表2的多条记录也可以对应表1的一条记录
 
一对一：
    表1的一条记录唯一对应表2的一条记录，反之亦然
 
分析时，我们先从按照上面的基本原理去套，然后再翻译成真实的意义，就很好理解了

辅助理解

#一对多或称为多对一
三张表：出版社，作者信息，书
 
一对多（或多对一）：一个出版社可以出版多本书
 
关联方式：foreign key

=====================多对一=====================
create table press(
id int primary key auto_increment,
name varchar(20)
);
 
create table book(
id int primary key auto_increment,
name varchar(20),
press_id int not null,
foreign key(press_id) references press(id)
on delete cascade
on update cascade
);
 
insert into press(name) values
('北京工业地雷出版社'),
('人民音乐不好听出版社'),
('知识产权没有用出版社')
;
 
insert into book(name,press_id) values
('九阳神功',1),
('九阴真经',2),
('九阴白骨爪',2),
('独孤九剑',3),
('降龙十巴掌',2),
('葵花宝典',3);

单张表：用户表+相亲关系表，相当于：用户表+相亲关系表+用户表
多张表：用户表+用户与主机关系表+主机表
 
中间那一张存放关系的表，对外关联的字段可以联合唯一

其它例子

#一对一
两张表：学生表和客户表
 
一对一：一个学生是一个客户，一个客户有可能变成一个学校，即一对一的关系
 
关联方式：foreign key+unique

例一：一个用户只有一个博客
 
    用户表：
    id  name
   egon
   alex
   wupeiqi
 
    博客表
           fk+unique
    id url name_id
 xxxx   1
 yyyy   3
 zzz    2
 
例二：一个管理员唯一对应一个用户
    用户表：
    id user  password
 egon    xxxx
 alex    yyyy
 
    管理员表：
       fk+unique
    id user_id password
  1      xxxxx
  2      yyyyy

其他例子

七 作业

练习：账号信息表，用户组，主机表，主机组

#用户表
create table user(
id int not null unique auto_increment,
username varchar(20) not null,
password varchar(50) not null,
primary key(username,password)
);
 
insert into user(username,password) values
('root',''),
('egon',''),
('alex','alex3714')
;
 
#用户组表
create table usergroup(
id int primary key auto_increment,
groupname varchar(20) not null unique
);
 
insert into usergroup(groupname) values
('IT'),
('Sale'),
('Finance'),
('boss')
;
 
#主机表
create table host(
id int primary key auto_increment,
ip char(15) not null unique default '127.0.0.1'
);
 
insert into host(ip) values
('172.16.45.2'),
('172.16.31.10'),
('172.16.45.3'),
('172.16.31.11'),
('172.10.45.3'),
('172.10.45.4'),
('172.10.45.5'),
('192.168.1.20'),
('192.168.1.21'),
('192.168.1.22'),
('192.168.2.23'),
('192.168.2.223'),
('192.168.2.24'),
('192.168.3.22'),
('192.168.3.23'),
('192.168.3.24')
;
 
#业务线表
create table business(
id int primary key auto_increment,
business varchar(20) not null unique
);
insert into business(business) values
('轻松贷'),
('随便花'),
('大富翁'),
('穷一生')
;
 
#建关系：user与usergroup
 
create table user2usergroup(
id int not null unique auto_increment,
user_id int not null,
group_id int not null,
primary key(user_id,group_id),
foreign key(user_id) references user(id),
foreign key(group_id) references usergroup(id)
);
 
insert into user2usergroup(user_id,group_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(2,3),
(2,4),
(3,4)
;
 
#建关系：host与business
 
create table host2business(
id int not null unique auto_increment,
host_id int not null,
business_id int not null,
primary key(host_id,business_id),
foreign key(host_id) references host(id),
foreign key(business_id) references business(id)
);
 
insert into host2business(host_id,business_id) values
(1,1),
(1,2),
(1,3),
(2,2),
(2,3),
(3,4)
;
 
#建关系：user与host
 
create table user2host(
id int not null unique auto_increment,
user_id int not null,
host_id int not null,
primary key(user_id,host_id),
foreign key(user_id) references user(id),
foreign key(host_id) references host(id)
);
 
insert into user2host(user_id,host_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(1,5),
(1,6),
(1,7),
(1,8),
(1,9),
(1,10),
(1,11),
(1,12),
(1,13),
(1,14),
(1,15),
(1,16),
(2,2),
(2,3),
(2,4),
(2,5),
(3,10),
(3,11),
(3,12);

作业：