create database exam; use exam; create table class( id int extends '编号', cname char(10) extends '姓名' ) extends '班级表'; create table student( sid int extends '编号', sname char(20) extends '姓名', classid int extends '班级', score float extends '成绩' ) extends '学生信息表'; select* from student; select sid,sname,score from student order sid asc; select avg(score) from student where classid=2;