2019独角兽企业重金招聘Python工程师标准>>> 
表的约束
主键约束
create table student(id int primary key,name varchar(40));
create table student(id int primary key auto_increment,name varchar(20));
唯一约束
create table student(id int primary key auto_increment,name varchar(20) u
nique);
非空约束
create table student(id int primary key auto_increment,name varchar(20) u
nique not null);
外键约束
create table teacher(id int primary key auto_increment,name varchar(20) no
t null,student_id int,constraint student_id_FK foreign key(student_id) reference
s student(id));
数据库的设计
一对一（一般有主从关系，把从的那个表的主键作外键约束）
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));
一对多或多对一（先设计表，在多的一方加外键）
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（先设计两个表，再设计中间表(可以设联合主键)）
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));
自连接
create table person(id int primary key,name varchar(40),parent_id int,constraint parent_id_FK foreign key(parent_id) references person(id));