mysql 添加外键

create table class(
cid tinyint unsigned primary key auto_increment,
caption varchar(15) not null
)engine=innodb;

create table student(
sid smallint unsigned primary key auto_increment,
sname varchar(15) not null,
gender enum('male','female') default 'male',
class_id tinyint unsigned,
constraint fk_name foreign key(class_id)
references class(cid)
on delete cascade
on update cascade
)engine=innodb;

create table teacher(
tid tinyint unsigned primary key auto_increment,
tname varchar(15) not null
)engine=innodb;

create table course(
cid smallint unsigned primary key auto_increment,
cname varchar(15) not null,
teacher_id tinyint unsigned,
constraint fk_teacher_id foreign key(teacher_id)
references teacher(tid)
on delete cascade
on update cascade
)engine=innodb;

create table score(
sid int unsigned primary key auto_increment,
number tinyint unique not null,
student_id smallint unsigned,
course_id smallint unsigned,
constraint fk_student foreign key(student_id)
references student(sid),
constraint fk_course foreign key(course_id)
references course(cid)
on delete cascade
on update cascade
)engine=innodb;