表——完整约束性规则(键)

时间:2022-04-23
本文章向大家介绍表——完整约束性规则(键),主要内容包括一 介绍、二 not null与default、三 unique、四 primary key、五 auto_increment、六 foreign key、ALTER TABLE 时的 SQL FOREIGN KEY 约束、撤销 FOREIGN KEY 约束、基本概念、基础应用、原理机制和需要注意的事项等,并结合实例形式分析了其使用技巧,希望通过本文能帮助到大家理解应用这部分内容。

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'    #设置了不能为空,为空时默认使用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( id 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 |
+------+-----+------+------------+

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)   # 给UNIQUE KEY设置名字:uk,不设置系统也会给它命一个命称
);


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'  #因为设置了唯一,但是赋值时重复了
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    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.00 sec)
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  host和port都相同时才报错
);

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'

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) ,有多列唯一的效果

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)
==================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    |       |
+--------------+-------------+------+-----+---------+-------+
3 rows in set (0.00 sec)

mysql> insert into service values
    -> ('172.16.45.10','3306','mysqld'),
    -> ('172.16.45.11','3306','mariadb')
    -> ;
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> insert into service values ('172.16.45.10','3306','nginx');    #重复了,报错
ERROR 1062 (23000): Duplicate entry '172.16.45.10-3306' for key 'PRIMARY'

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 |
+----+------+------+
1 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 |
+----+-------+------+

六 foreign key

CREATE TABLE 时的 SQL FOREIGN KEY 约束

下面的 SQL 在 "Orders" 表创建时在 "P_Id" 列上创建 FOREIGN KEY 约束:

MySQL:

CREATE TABLE Orders ( O_Id int NOT NULL, OrderNo int NOT NULL, P_Id int, PRIMARY KEY (O_Id), FOREIGN KEY (P_Id) REFERENCES Persons(P_Id) )

SQL Server / Oracle / MS Access:

CREATE TABLE Orders ( O_Id int NOT NULL PRIMARY KEY, OrderNo int NOT NULL, P_Id int FOREIGN KEY REFERENCES Persons(P_Id) )

如需命名 FOREIGN KEY 约束,并定义多个列的 FOREIGN KEY 约束,请使用下面的 SQL 语法:

MySQL / SQL Server / Oracle / MS Access:

CREATE TABLE Orders ( O_Id int NOT NULL, OrderNo int NOT NULL, P_Id int, PRIMARY KEY (O_Id), CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id) REFERENCES Persons(P_Id) )

ALTER TABLE 时的 SQL FOREIGN KEY 约束

当 "Orders" 表已被创建时,如需在 "P_Id" 列创建 FOREIGN KEY 约束,请使用下面的 SQL:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders ADD FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)

如需命名 FOREIGN KEY 约束,并定义多个列的 FOREIGN KEY 约束,请使用下面的 SQL 语法:

MySQL / SQL Server / Oracle / MS Access:

ALTER TABLE Orders ADD CONSTRAINT fk_PerOrders FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)

撤销 FOREIGN KEY 约束

如需撤销 FOREIGN KEY 约束,请使用下面的 SQL:

MySQL:

ALTER TABLE Orders DROP FOREIGN KEY fk_PerOrders

SQL Server / Oracle / MS Access:

ALTER TABLE Orders DROP CONSTRAINT fk_PerOrders

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

#!!!先建被关联的表,并且被关联的字段必须唯一
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 |
+----+-------+--------+
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
#多对一或称为一对多
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

建立多对一的关系需要注意
1 先建立被关联的表,被关联的字段必须保证是唯一的
2 再创建关联的表,关联的字段,一定要保证是可以重复的
ps:关联的字段一定是来自于表关联的表对应字段的值
=====================多对一=====================
先建立被关联的表

create table press(
id int primary key auto_increment,    #被关联的字段必须保证是唯一的
name varchar(20)
);


再创建关联的表

create table book(
id int primary key auto_increment,
name varchar(20),
press_id int not null,    #关联的字段,一定要保证是可以重复的
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),    #关联的字段一定是来自于表关联的表对应字段的值
('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)
;
#多对多
三张表:出版社,作者信息,书

多对多:一个作者可以写多本书,一本书也可以有多个作者,双向的一对多,即多对多
  
关联方式:foreign key+一张新的表
=====================多对多=====================
create table author(
id int primary key auto_increment,
name varchar(20)
);


#这张表就存放作者表与书表的关系,即查询二者的关系查这表就可以了

create table author2book(
id int primary key auto_increment,
author_id int not null,
book_id int not null,
foreign key(author_id) references author(id) on delete cascade on update cascade,
foreign key(book_id) references book(id)
on delete cascade
on update cascade,
unique (author_id,book_id)
);


#插入四个作者,id依次排开
insert into author(name) values('egon'),('alex'),('yuanhao'),('wpq');

#每个作者与自己的代表作如下
1 egon: 
      1 九阳神功
      2 九阴真经
      3 九阴白骨爪
      4 独孤九剑
      5 降龙十巴掌
      6 葵花宝典


2 alex: 
      1 九阳神功
      6 葵花宝典

3 yuanhao:
      4 独孤九剑
      5 降龙十巴掌
      6 葵花宝典

4 wpq:
      1 九阳神功


insert into author2book(author_id,book_id) values
(1,1),
(1,2),
(1,3),
(1,4),
(1,5),
(1,6),
(2,1),
(2,6),
(3,4),
(3,5),
(3,6),
(4,1)
;
#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
#一定是student来foreign key表customer,这样就保证了:
#1 学生一定是一个客户,
#2 客户不一定是学生,但有可能成为一个学生


create table customer(
id int primary key auto_increment,
name varchar(20) not null
);


create table student(
id int primary key auto_increment,
name varchar(20) not null,
class_name varchar(20) not null default 'python自动化',
level int default 1,
customer_id int unique, #该字段一定要是唯一的
foreign key(customer_id) references customer(id) #外键的字段一定要保证unique
on delete cascade
on update cascade
);


#增加客户
insert into customer(name) values
('李飞机'),
('王大炮'),
('守榴弹'),
('吴坦克'),
('赢火箭'),
('战地雷')
;


#增加学生
insert into student(name,customer_id) values
('李飞机',1),
('王大炮',2)
;

多对一(一对多)、一对一、多对多的关系:

foreign key(表1_字段名) references 表2(字段名);

在表2的字段名是主键的情况下(即不为空,且唯一)

若表1的字段名不唯一,则是多对一

若表1的字段名唯一(unique,则是一对一

多对多则需要另建一个表来单独存放多对多的关系(即有关联的字段),

foreign key(新表_字段名1) references 表1(字段名)  on delete cascade on update cascade,

foreign key(新表_字段名2) references 表1(字段名) on delete cascade on update cascade;

外键对应的字段数据类型必须一致,两张表的存储引擎必须一致,否则会出错