练习：账号信息表，用户组，主机表，主机组

#用户表
create table user(
id int not null unique auto_increment,
username varchar(20) not null,
password varchar(50) not null,
primary key(username,password)
);

#用户组表
create table usergroup(
id int primary key auto_increment,
groupname varchar(20) not null unique
);

#主机表
create table host(
id int primary key auto_increment,
ip char(15) not null unique default ‘127.0.0.1‘
);

#业务线表
create table business(
id int primary key auto_increment,
business varchar(20) not null unique
);

#建关系：user与usergroup

create table user2usergroup(
id int not null unique auto_increment,
user_id int not null,
group_id int not null,
primary key(user_id,group_id),
foreign key(user_id) references user(id),
foreign key(group_id) references usergroup(id)
);

#建关系：host与business
create table host2business(
id int not null unique auto_increment,
host_id int not null,
business_id int not null,
primary key(host_id,business_id),
foreign key(host_id) references host(id),
foreign key(business_id) references business(id)
);

#建关系：user与host
create table user2host(
id int not null unique auto_increment,
user_id int not null,
host_id int not null,
primary key(user_id,host_id),
foreign key(user_id) references user(id),
foreign key(host_id) references host(id)
);

练习2:学校数据库的表建立

# 班级表
cid	caption
# 学生表
sid sname gender class_id
# 老师表
tid	tname
# 课程表
cid	cname	teacher_id
# 成绩表
sid	student_id course_id number

# 创建学校数据库
create database school;
# 进入学校库
use school;
# 创建班级表
create table class(
	cid int,
    caption varchar(64)
)
# 创建学生表
# 学生表与班级表为一对多的关系，且学生表为多的那一边
create table student(
	sid int primary key auto_increment,
    sname varchar(32),
	gender enum("male","female","others") default"male",
    class_id int not null,
    foreign key(class_id) references class(cid)
    on update cascade
    on delete cascade
);
# 创建老师表
create table teacher(
	tid int primary key,	
	tname varchar(64)
);
# 创建课程表
# 老师与课程一对多，一个老师可以教多门课
create table course(
	cid int primary key,
    cname varchar(32),
    teacher_id int not null,
    foreign key(teacher_id) references teacher(tid)
    on update cascade
    on delete cascade
);
# 成绩表
# student_id course_id 应为联合唯一
create table score(
	sid int primary key,
    student_id int not null,
    course_id int not null,
    number int not null,
    foreign key(student_id) references student(sid)
    on update cascade
    on delete cascade,
    foreign key(course_id) references course(cid)
    on update cascade
    on delete cascade,
    unique(student_id,course_id)
);

5.5---MySQL创建相互关联的表练习