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MySQL之约束

爱做梦的老巫婆 2022-04-07 阅读 72
mysql

文章目录

MySQL之约束

注:本文是以Windos MySQL v8.0.25进行讲解

1.约束分类

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2.演示(非空约束not null)

--添加非空约束第一种方式:创建表时添加非空约束
		CREATE TABLE per(
			id INT,
			NAME VARCHAR(20) NOT NULL -- name为非空
		);
--添加非空约束第二种方式:创建表完后才添加非空约束
		ALTER TABLE per MODIFY NAME VARCHAR(20) NOT NULL;-- 注:modify的意思是更改、修改的意思

--删除name的非空约束
		ALTER TABLE per MODIFY NAME VARCHAR(20);

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3.演示(唯一约束unique)

--添加唯一约束第一种方式:在创建表时,添加唯一约束
		CREATE TABLE per(
			id INT,
			phone_number VARCHAR(20) UNIQUE -- 手机号
		);
--添加唯一约束第二种方式:在表创建完后,添加唯一约束
		ALTER TABLE per MODIFY phone_number VARCHAR(20) UNIQUE;	

--删除唯一约束
		ALTER TABLE per DROP INDEX phone_number; -- 删除方式的语法比较特殊

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4.演示(主键约束primary key)

--添加主键约束第一种方式:在创建表时添加主键约束
		create table per(
			id int primary key,-- 给id添加主键约束
			name varchar(20)s
		);
--添加主键约束第二种方式:创建完表后,添加主键
		ALTER TABLE per MODIFY id INT PRIMARY KEY;
--删除主键:
		ALTER TABLE per DROP PRIMARY KEY;--注:删除主键后,id仍然不能为null

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6.演示(主键约束primary key自动增长)


--添加自动增长第一种方式:在创建表时,添加主键约束,并且完成主键自增长
		create table per(
			id int primary key auto_increment,-- 给id添加主键约束
			name varchar(20)
		);
--添加自动增长第二种方式: 添加自动增长
		ALTER TABLE per MODIFY id INT AUTO_INCREMENT;
--删除自动增长
		ALTER TABLE per MODIFY id INT;--注:删除完自动增长后,主键仍然后还在

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7.演示(外键约束前言)

CREATE TABLE emp (
id INT PRIMARY KEY AUTO_INCREMENT,
NAME VARCHAR(30),
age INT,
dep_name VARCHAR(30),
dep_location VARCHAR(30)
);-- 添加数据
INSERT INTO emp (NAME, age, dep_name, dep_location) VALUES ('张三', 20, '研发部', '广州');
INSERT INTO emp (NAME, age, dep_name, dep_location) VALUES ('李四', 21, '研发部', '广州');
INSERT INTO emp (NAME, age, dep_name, dep_location) VALUES ('王五', 20, '研发部', '广州');
INSERT INTO emp (NAME, age, dep_name, dep_location) VALUES ('老王', 20, '销售部', '深圳');
INSERT INTO emp (NAME, age, dep_name, dep_location) VALUES ('大王', 22, '销售部', '深圳');
INSERT INTO emp (NAME, age, dep_name, dep_location) VALUES ('小王', 18, '销售部', '深圳');

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create table department(
id int primary key auto_increment,
dep_name varchar(20),
dep_location varchar(20)
);

create table employee(
id int primary key auto_increment,
name varchar(20),
age int,
dep_id int -- 外键对应主表的主键
);

insert into department values(null, '研发部','广州'),(null, '销售部', '深圳');
select * from department;
INSERT INTO employee (NAME, age, dep_id) VALUES ('张三', 20, 1);
INSERT INTO employee (NAME, age, dep_id) VALUES ('李四', 21, 1);
INSERT INTO employee (NAME, age, dep_id) VALUES ('王五', 20, 1);
INSERT INTO employee (NAME, age, dep_id) VALUES ('老王', 20, 2);
INSERT INTO employee (NAME, age, dep_id) VALUES ('大王', 22, 2);
INSERT INTO employee (NAME, age, dep_id) VALUES ('小王', 18, 2);
select * from employee;

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8.演示(外键约束primary key)

--添加外键约束第一种方式:在创建表时,可以添加外键
			create table 表名(
				....
				外键列
				constraint 外键名称 foreign key (外键列名称) references 主表名称(主表列名称)
			);
			 --注意:“外键名称”自己可以随便起名,constraint是限定与约束的意思
			      	
 --添加外键约束第二种方式:创建表之后,添加外键
  ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段名称) REFERENCES 主表名称(主表列名称);
      --删除外键:
		ALTER TABLE 表名 DROP FOREIGN KEY 外键名称;
	--注:有外键存在的时候必须得先删除employee,然后再删department,这样才能把两个表都给删了;如果先删除department表再删除employee表格的话则会失败
--忘记外键名称的查看方法:
show create table employee;
create table department(
id int primary key auto_increment,
dep_name varchar(20),
dep_location varchar(20)
);

create table employee(
id int primary key auto_increment,
name varchar(20),
age int,
dep_id int ,
constraint emp_dept_fk foreign key (dep_id) references department(id)
);

insert into department values(null, '研发部','广州'),(null, '销售部', '深圳');
select * from department;
INSERT INTO employee (NAME, age, dep_id) VALUES ('张三', 20, 1);
INSERT INTO employee (NAME, age, dep_id) VALUES ('李四', 21, 1);
INSERT INTO employee (NAME, age, dep_id) VALUES ('王五', 20, 1);
INSERT INTO employee (NAME, age, dep_id) VALUES ('老王', 20, 2);
INSERT INTO employee (NAME, age, dep_id) VALUES ('大王', 22, 2);
INSERT INTO employee (NAME, age, dep_id) VALUES ('小王', 18, 2);
select * from employee;

show create table employee;
alter table employee drop foreign key emp_dept_fk;

alter table employee add constraint emp_dept_fk foreign key (dep_id) references department(id);

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9.演示(传统级联更新)

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10.高级(传统级联更新)

		--添加级联操作语法:
			ALTER TABLE 表名 ADD CONSTRAINT 外键名称  FOREIGN KEY (外键字段名称) REFERENCES 主表名称(主表列名称) ON UPDATE CASCADE ON DELETE CASCADE  ;
		
			--级联更新:
			ON UPDATE CASCADE 
         --级联删除:
         ON DELETE CASCADE 
alter table employee add constraint emp_dept_fk foreign key (dep_id) references department(id) on update cascade;

alter table employee add constraint emp_dept_fk foreign key (dep_id) references department(id) on update cascade on delete cascade;

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11.多表之间的关系分类

一对一关系实现

一对多关系实现

多对多关系实现

12.演示(多表之间的关系分类)

CREATE TABLE tab_category (
			cid INT PRIMARY KEY AUTO_INCREMENT,  
			cname VARCHAR(100) NOT NULL UNIQUE
		);
				
CREATE TABLE tab_route(
			rid INT PRIMARY KEY AUTO_INCREMENT, 
			rname VARCHAR(100) NOT NULL UNIQUE,
			price DOUBLE,
			rdate DATE,
			cid INT,
			FOREIGN KEY (cid) REFERENCES tab_category(cid) #没有些定义外键名称,也就不应该写constraint ,系统会为我们自动分配一个外键名称;  tab_
		);
				
CREATE TABLE tab_user (
			uid INT PRIMARY KEY AUTO_INCREMENT,
			username VARCHAR(100) UNIQUE NOT NULL,
			PASSWORD VARCHAR(30) NOT NULL,
			NAME VARCHAR(100),
			birthday DATE,
			sex CHAR(1) DEFAULT '男',
			telephone VARCHAR(11),
			email VARCHAR(100)
		);
		
CREATE TABLE tab_favorite (
			rid INT, 
			DATE DATETIME,
			uid INT, 
			PRIMARY KEY(rid,uid), #创建复合主键,即联合主键,rid与uid合起来为tab_favorite的一个主键
			FOREIGN KEY (rid) REFERENCES tab_route(rid),
			FOREIGN KEY(uid) REFERENCES tab_user(uid)
		);

select * from  tab_category;
select * from  tab_route;
select * from  tab_user;
select * from  tab_favorite;

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13.超键、候选键、主键、外键分别是什么

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