mysql (one-to-one, one-to-many, many-to-many)

One, one to one

 

  create table employee ( 
  eno int primary key auto_increment comment'employee number', 
  ename char(10) comment'username', 
  password int comment'password');

 create table emp ( 
 eno int primary key auto_increment comment'employee number', 
 ename char(10) comment'name', 
 sex char(2) comment'gender', 
 tele int comment'telephone', 
 email varchar(10) comment'mailbox ', 
 foreign key (eno) references employee (eno));

2. One-to-many

 create table class( 
 cno int primary key comment 'class number', 
 cname char(10) comment 'class name' 
 ); 

 create table student( 
 sno int primary key comment 'student number', 
 sname char(10) comment 'student name' , 
 sex char(2) comment 'gender', 
 tele int comment 'telephone', 
 cno int comment 'class number', 
 foreign key (cno) references class(cno) 
 );

1. Number of people inquiring about computer and e-commerce studies

 select cname,count(sname) from student,class where student.cno = class.cno group by cname;+----------+--------------+ | 

cname |    count(sname) |

Computer 2 

E-commerce 2 

3. Many-to-many

 create table uesr( 
 uno int primary key comment 'user number', 
 uname char(10) comment 'username', 
 password int comment 'password' 
 );

 create table role( 
  rno int primary key comment 'role number', 
  rname char(10) comment 'role name' 
 );

 create table tee(
 id int primary key,
 uno int,
 rno int,
 foreign key (uno) references uesr(uno),
 foreign key (rno) references role(rno)
 );