MYSQL第五章节有关约束操作详解（附代码，例题详解）这一篇就够了

CREATE TABLE user(
id int  PRIMARY KEY auto_increment,
name varchar(10) not null unique ,
age int check (age>0&&age<=120),
status char(1) default '1',
gender char(1)
);

-- 方法一 --
CREATE TABLE 表名(
 
    字段名 数据类型,
    ...
    [CONSTRAINT] [外键名字] FOREIGN KEY(外键字段名) REFERENCES 主表(主表列名)
 
);
 
 
-- 方法二 --
 
ALTER TABLE 表名 ADD CONSTRAINT 外键名 FOREIGN KEY(外键字段名) REFERENCES 主表(主表列名);
 
 

CREATE TABLE user(
id int  PRIMARY KEY auto_increment,
name varchar(10) not null unique ,
age int check (age>0&&age<=120),
status char(1) default '1',
gender char(1)
);
desc user
 
insert into user(name,age,status,gender) values ('Tom1',19,'1','男'),('Tom2',25,'0','男');
insert into user(name,age,status,gender) values ('Tom3',19,'1','男');
 
insert into user(name,age,status,gender) values (null,19,'1','男');
insert into user(name,age,status,gender) values ('Tom3',19,'1','男');
 
insert into user(name,age,status,gender) values ('Tom4',80,'1','男');
insert into user(name,age,status,gender) values ('Tom5',-1,'1','男');
insert into user(name,age,status,gender) values ('Tom5',121,'1','男');
 
insert into user(name,age,gender) values ('Tom5',120,'男');
 
create table dept(
    id   int auto_increment comment 'ID' primary key,
    name varchar(50) not null comment '部门名称'
)comment '部门表';
INSERT INTO dept (id, name) VALUES (1, '研发部'), (2, '市场部'),(3, '财务部'), (4, '销售部'), (5, '总经办');
 
create table emp(
    id  int auto_increment comment 'ID' primary key,
    name varchar(50) not null comment '姓名',
    age  int comment '年龄',
    job varchar(20) comment '职位',
    salary int comment '薪资',
    entrydate date comment '入职时间',
    managerid int comment '直属领导ID',
    dept_id int comment '部门ID'
)comment '员工表';
 
INSERT INTO emp (id, name, age, job,salary, entrydate, managerid, dept_id) VALUES
            (1, '金庸', 66, '总裁',20000, '2000-01-01', null,5),(2, '张无忌', 20, '项目经理',12500, '2005-12-05', 1,1),
            (3, '杨逍', 33, '开发', 8400,'2000-11-03', 2,1),(4, '韦一笑', 48, '开发',11000, '2002-02-05', 2,1),
            (5, '常遇春', 43, '开发',10500, '2004-09-07', 3,1),(6, '小昭', 19, '程序员鼓励师',6600, '2004-10-12', 2,1);

ALTER TABLE emp ADD CONSTRAINT fk_emp_dept foreign key (dept_id) references dept(id);

ALTER TABLE 表名 DROP FOREIGN KEY 外键名字;

ALTER TABLE 表名 ADD CONSTRAINT (外键字段) REFERENCES 主表名(主表字段名) ON UPDATE CASCADE ON DELETE CASCADE;