Mysql-完整性约束

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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\'
View Code
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)
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    |       |
+--------------+-------------+------+-----+---------+-------+
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\');
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 |
+----+------+------+
1 row in set (0.00 sec)
View Code
#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

  •  快速理解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,越浪费存储空间

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,让员工信息表部门部分存储为部门ID-------可以解释为ID为变量名,实际名称为部门名字.

如何关联,即foreign key(关键字)

#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(变量表必须是唯一的)
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,\'技术发展事业部\'), (2,\'艾人力资源部\'), (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\',1), (7,\'张火箭\',3), (8,\'林子弹\',1), (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 | +----+-------+--------+
  • 如何找出两张表之间的关系 (多对一 & 一对多 & 多对多)
暂时未建立相应的表,两个表之间有相应的归属关系.比如:一个表中有另一个表的字段.字段值必须是单个的,不能是是多个值
如果两个表没啥关系,也就没有所谓一对一或者多对一的关系了.

分析步骤:
#1、先站在左表的角度去找 左表 员工 右表 部门 是否左表的多条记录的某字段值可以对应右表的一条记录,如果是,则证明左表的一个字段foreign key 右表一个字段(通常是id) #2、再站在右表的角度去找 是否右表的多条记录的某字段值可以对应左表的一条记录,如果是,则证明右表的一个字段foreign key 左表一个字段(通常是id) #3、总结: #多对一: 一 的关键信息必须是不能重复的 如果只有步骤1成立,则是左表多对一右表 如果只有步骤2成立,则是右表多对一左表 #多对多 老师和班级 如果步骤1和2同时成立,则证明这两张表时一个双向的多对一,即多对多,需要定义一个这两张表的关系表来专门存放二者的关系 #一对一: 如果1和2都不成立,而是左表的一条记录唯一对应右表的一条记录,反之亦然。这种情况很简单,就是在左表foreign key右表的基础上,将左表的外键字段设置成unique即可

三 建立表之间的关系

#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)
;
View Code

 

 

 

#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 not null unique auto_increment,
author_id int not null,
book_id int not null,
constraint fk_author foreign key(author_id) references author(id)
on delete cascade
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