use school
create table teach_course(
    course_id char(4),cname varchar(30),teacher_id char(6)
    constraint PK_teach_course primary key(course_id)
    constraint FK_teach_course foreign key(teacher_id) 
                               references listen_course(teacher_id)
) 
alter table listen_course
      add constraint FK_listen_course foreign key(course_id) 
                     references teach_course(course_id);