order by 和 ground by 区别：

order by 排序查询：asc升序 desc降序
select * from Student order by age根据学生的年龄查询学生信息（默认是升序）
ground by:分组查询 having只能用于ground by子句作用于组内

1. 按照学号分组查询每个学号的总成绩
   select 学号, SUM(成绩) from 选课表 ground by 学号
2. 查询平均成绩大于001课程平均成绩的学号并按平均成绩的降序排列
   select 学号,SUM(成绩) from 成绩表 ground by 学号 having AVG(成绩) >(select AVG(成绩) from 选课表 where 课程号=‘001’) order by AVG(成绩) desc;