mysql 查询表中平均分最低的班级

drop table if exists dd; 
create table dd ( 
user_id int , 
class_no int , 
score int 
); 
insert into dd values (1,1,1), (2,1,1), (3,1,2), (4,2,2); 
select class_no ,avg(score)from dd group by class_no order by avg(score);