作业5月5号

# 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

```mysql
# 课程表被学生表 班级表 学生表 老师表关联
create table course(
    course_id int primary key auto_increment,
    course_name varchar(32)
);
insert into course(course_name) values('english'),('math');


# 班级表是被学生表与课程表关联的
create table class(
    cid int primary key auto_increment,
    class_name varchar(18) not null
)；
insert into class(class_name) values('python'),('linux'),('java');

# 班级与课程关联表
create table class2course(
    rid int primary key auto_increment,
    class_id int,
    course_id int,
    
    foreign key(class_id) references class(cid)
    on update cascade 
    on delete cascade,
    
    foreign key(course_id) references course(course_id)
    on update cascade 
    on delete cascade
);
insert into class2course(class_id,course_id) values(1,2),(2,1),(3,2),(1,1);

# 成绩表关联学生表
create table score(
    score_id int primary key auto_increment,
    english_score int default 0,
    math_score int default 0
);
insert into score values(),(),();

# 学生表关联班级表
create table student(
    sid int primary key auto_increment,
    sname char(16),
    sage int default 18,
    class_id int,
    score_id int unique,
    
    foreign key(class_id) references class(cid)
    on update cascade
    on delete cascade,
    
    foreign key(score_id) references score(score_id)
    on update cascade
    on delete cascade 
);
insert into student(sname,class_id,score_id) values('tank',1,2),('egon',2,1),('jason',3,3);

create table teacher(
    tid int primary key auto_increment,
    name char(16),
    course_id int,
    
    foreign key(course_id) references course(course_id)
    on update cascade
    on delete cascade
);
insert into teacher(name,course_id) values('tank',1),('egon',1),('alex',2);