定义主键约束（每一个表必须有一个主键列）
create table student
(
id int  primary key,
name varchar(40)
);

定义主键自动增长
create table student
(
id int  primary key auto_increment,
name varchar(40)
);

定义唯一约束
drop table student;
create table student
(
id int primary key auto_increment,
name varchar(40) unique
);

定义非空约束
drop table student;
create table student
(
id int primary key auto_increment,
name varchar(40) unique not null
);

定义外键约束
create table husband
(
id int primary key,
name varchar(40)
);

create table wife
(
id int primary key,
name varchar(40),
husband_id int,
constraint husband_id_FK foreign key(husband_id) references husband(id)
);

一对多或多对一的对象存到数据库时，表的设计方案
部门和员工
create table department
(
id int primary key,
name varchar(40)
);

create table employee
(
id int primary key,
name varchar(40),
salary decimal(8,2),
department_id int,
constraint department_id_FK foreign key(department_id) references department(id)
);

多对多对象的表的设计（老师和学生）
create table teacher
(
id int primary key,
name varchar(40),
salary decimal(8,2)
);

create table student
(
id int primary key,
name varchar(40)
);

create table teacher_student
(
teacher_id int,
student_id int,
primary key(teacher_id,student_id),
constraint teacher_id_FK foreign key(teacher_id) references teacher(id),
constraint student_id_FK foreign key(student_id) references student(id)
);

一对一的对象的数据库设计
create table person
(
id int primary key,
name varchar(40)
);

create table idcard
(
id int primary key,
city varchar(40),
constraint id_FK foreign key(id) references person(id)
);

自连接的表
create table person
(
id int primary key,
name varchar(40),
parent_id int,
constraint parent_id_FK foreign key(parent_id) references person(id)
);