完整性约束
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
分类:
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203. 是否是key主键 primary key外键 foreign key索引 (index,unique...)
二、not null 与 default
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) ); 方法三: create table department( id int, name char(10), unique(id), unique(name) ); 方法四: 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 几个字段和在一起不重复 ip,port和在一起唯一 ); not null + unique 的化学反应 = primary key
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unique
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:不为空且唯一; 多列做主键(复合主键):多个字段连在一起,当作主键; 存储引擎: 默认 innodb,对于innodb存储引擎来说,一张表内容必须有一个主键; 主键: 功能,不为空且唯一,还有其他方面得功能,组织表得数据,提高查询方式; 主键: 如果不指定主键,innodb会找字段不为空且唯一得字段把它设为主键,如果没找到,就会有隐藏得字段为主键, 用主键来组织数据,拿到主键加速查找。建主键,可以加速查找,innodb 应该有个自己建得主键。 这是innodb 独特得特性,和innodb 组织数据得方式有关 注: 通常一张表,都应该有一个id字段用来标识记录编号,id通常设置为主键。 一个表内只能有一个主键primary key
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) ); 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) ); 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 ); 单列做主键
==================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','3306','mysqld'), -> ('172.16.45.11','3306','mariadb') -> ;Query OK, 2 rows affected (0.00 sec)Records: 2 Duplicates: 0 Warnings: 0mysql> 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
1.不指定id,则自动增长
#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 |+----+------+------+
2.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 |+----+------+--------+
3.问题
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 |+----+------+------+
4.解决方法
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)
5.字段的起始偏移量与步长
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=utf8mysql> 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
#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=utf8mysql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=2mysql> 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
注意
- auto_increment时增加新内容
- 清空表:
- delete from t1; #如果有自增id,新增的数据,仍然是以删除前的最后一样作为起始。
- truncate table t1;数据量大,删除速度比上一条快,且直接从零开始
六、foreign key
一 、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
- 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 ke
1.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; 2.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 # 关联表会受到影响,删除被关联表时,关联得表会自动得删。 on update cascade # 关联表会受到影响,修改被关联表时,关联得表会自动得修改。 )engine=innodb; 3.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'); 4.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); 5.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 | +----+-------+--------+ 6.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 | +----+-------+--------+快速理解foreign key
二、如何找出两张表之间得关系:
分析步骤:#1、先站在左表的角度去找是否左表的多条记录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: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:
1.多对一或一对多
#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
#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 cascadeon 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);
2.多对多
#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 cascadeon update cascade,constraint fk_book foreign key(book_id) references book(id)on delete cascadeon update cascade,primary key(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egon: 九阳神功九阴真经九阴白骨爪独孤九剑降龙十巴掌葵花宝典alex: 九阳神功葵花宝典yuanhao:独孤九剑降龙十巴掌葵花宝典wpq: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);
3.一对一
#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
#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,qq varchar(10) not null,phone char(16) not null);create table student(id int primary key auto_increment,class_name varchar(20) not null,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uniqueon delete cascadeon update cascade);#增加客户insert into customer(name,qq,phone) values('李飞机','31811231',13811341220),('王大炮','123123123',15213146809),('守榴弹','283818181',1867141331),('吴坦克','283818181',1851143312),('赢火箭','888818181',1861243314),('战地雷','112312312',18811431230);#增加学生insert into student(class_name,customer_id) values('脱产3班',3),('周末19期',4),('周末19期',5);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
2.如何找出两张表之间得关系: