表完整性约束
作用,用于保证数据的完整性和一致性
|约束条件|说明|
|-|-|
|primary key|标识该字段为该表的主键,可以唯一的标识记录,不可以为空|
|foreing key|标识该字段为表的外键,实现表与表之间的关联|
|not null|标识该字段不能为空|
|unique key|标识该字段的值是唯一的,可以为空,一个表可以有多个unique key|
|auto_increment|标识该字段的值是自动增长(整数类型,而且是主键)|
|default|为该字段设置默认值|
|unsigned|无符号,正数|
|zerofill|使用0填充,例如000000001|
default(默认值)和not null(是否允许空值)
- 默认值不允许为函数
- NULL值就是没有值或缺值
- 允许NULL值的列，就是允许在插入行时不给出该列的值
- 不允许NULL值的列在插入或更新行时，该列必须有值
- 不要把NULL值与空串相混淆。NULL值是没有值，它不是空串。
- 如果指定‘’两个单引号，其间没有字符，这在NOT NULL列是允许的
mysql> use test1; Database changed mysql> create table student4( -> id int not null, -> name varchar(50) not null, -> sex enum('m','f') default 'm' not null, -> age int unsigned default 18 not null, -> hobby set('disc','book','music') default 'disc,book' not null); Query OK, 0 rows affected (0.19 sec)mysql> desc student4; +-------+----------------------------+------+-----+-----------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+----------------------------+------+-----+-----------+-------+ | id | int(11) | NO | | NULL | | | name | varchar(50) | NO | | NULL | | | sex | enum('m','f') | NO | | m | | | age | int(10) unsigned | NO | | 18 | | | hobby | set('disc','book','music') | NO | | disc,book | | +-------+----------------------------+------+-----+-----------+-------+ 5 rows in set (0.00 sec)unique key(唯一)
可以为空,但是不允许重复
mysql> create table department1( -> dept_in int, -> dept_name varchar(50) unique, -> dept_comment varchar(100) );
unique可以在列名的最后指定,也可以在最后指定
mysql> create table department2( -> id int, -> name varchar(50), -> comment varchar(100), -> unique(name));
AUTO_INCREMENT
AUTO_INCREMENT告诉mysql，本列每增加一行时自动增量，每次执行一个INSERT操作时，MYSQL自动对该列增量（从而才有了AUTO_INCREMENT），给该列赋予下一个可用的值，这样给每个行分配一个唯一的cust_id,从而可以用作主键值
- 每个表只允许一个AUTO_INCREMENT列，而且它必须被索引（通常使它成为主键）
可以通过SELECT last_insert_id()语句来获取最后一个AUTO_INCREMENT
主键
- 单列做主键
多列做主键(复合主键)
primary key字段的值是不允许重复,且不允许为null
mysql> create table student6(
-> id int primary key not null auto_increment,
-> name varchar(50) not null,
-> sex enum('male','female') not null default 'male',
-> age int not null default 18);
Query OK, 0 rows affected (0.18 sec)
插入数据
mysql> insert into student6(name,sex,age) values
-> ('jack','male',17),
-> ('alice','female',23),
-> ('tianyun','male',40);
Query OK, 3 rows affected (0.01 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from student6; +----+---------+--------+-----+ | id | name | sex | age | +----+---------+--------+-----+ | 1 | jack | male | 17 | | 2 | alice | female | 23 | | 3 | tianyun | male | 40 |
主键不能重复
mysql> insert into student6 values(2,'harry','male',17); ERROR 1062 (23000): Duplicate entry '2' for key 'PRIMARY'
复合主键
场景:单列不能证明唯一性,需要使用多列创建
|设置主键为:|host_ip+port=primary key)|
|-|-|
|host_ip | 存储主机IP|
|service_name | 服务名|
|port | 服务对应的端口|
|allowy(y,n) | 服务是否允许访问|
mysql> create table service(
-> ip varchar(15) not null,
-> service varchar(16) not null,
-> port int not null,
-> pritocol enum('tcp','udp') default 'tcp',
-> allow enum('y','n') default 'n',
-> primary key(ip,port));
插入数据
mysql> insert into service values('192.168.122.223','http',80,'tcp','y');
Query OK, 1 row affected (0.04 sec)
mysql> insert into service(ip,service,port) values('192.168.122.223','ftp',21);
Query OK, 1 row affected (0.02 sec)
mysql> insert into service values('192.168.122.220','http',80,'tcp','y');
Query OK, 1 row affected (0.03 sec)
mysql> select * from service; +-----------------+---------+------+----------+-------+ | ip | service | port | pritocol | allow | +-----------------+---------+------+----------+-------+ | 192.168.122.220 | http | 80 | tcp | y | | 192.168.122.223 | ftp | 21 | tcp | n | | 192.168.122.223 | http | 80 | tcp | y | +-----------------+---------+------+----------+-------+ 3 rows in set (0.00 sec)
foreign key(外键)
存储引擎必须是innodb
用子表的外键关联父表的主键,这两个字段的名称,数据类型和约束都需要相同
- foreign key(name) ---- 设置当前表的外键为name
- references employees(name) ----关联employees表的name字段
- on update cascade ---- 同步更新
- on delete cascade ---- 同步删除
设置父表company.employees
mysql> create table employees(
-> name varchar(50) not null,
-> mail varchar(20),
-> primary key(name));
Query OK, 0 rows affected (0.17 sec)
设置字表company.payroll
mysql> create table payroll(
-> id int auto_increment,
-> name varchar(50) not null,
-> payroll float(8,2) not null,
-> primary key(id),
-> foreign key(name) references employees(name) on update cascade on delete cascade);
Query OK, 0 rows affected (0.24 sec)
向父表员工表中插入值
mysql> desc employees; +-------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-------+-------------+------+-----+---------+-------+ | name | varchar(50) | NO | PRI | NULL | | | mail | varchar(20) | YES | | NULL | |
mysql> insert into employees values
-> ('jack','jack@163.com'),('alice',null);
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0
向子表薪水表中插入数据
mysql> insert into payroll(name,payroll) values('alice',20000);
Query OK, 1 row affected (0.03 sec)
mysql> insert into payroll(name,payroll) values('tianyun',25300);
ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails (`company`.`payroll`, CONSTRAINT `payroll_ibfk_1` FOREIGN KEY (`name`) REFERENCES `employees` (`name`) ON DELETE CASCADE ON UPDATE CASCADE)
发现员工表中没有tianyun,所有在薪水表中无法添加
主表中更改name名称,发现子表中name值也改变了
mysql> update employees set name='alice123' where name='alice'; Query OK, 1 row affected (0.01 sec) Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from payroll; +----+----------+----------+ | id | name | payroll | +----+----------+----------+ | 2 | alice123 | 20000.00 | +----+----------+----------+ 1 row in set (0.00 sec)
删除员工表中一位员工,发现新水表中该员工也会自动删除
mysql> delete from employees where name='alice123'; Query OK, 1 row affected (0.01 sec) mysql> select * from payroll; Empty set (0.00 sec)