查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列。

table 结构

create table course (    id int unsigned not null auto_increment,    course_name varchar(32) not null default '',    course_score float(5, 2) not null default 0.00,    primary key(id))engine=innodb default charset=utf8;

Res:

• select avg(course_score), course_name from course group by course_name avg(course_score) asc, id desc having avg(course_score) > 0;