group by 与 order by

group by

1.对重复的数据合并分组,
(group by a,b  a列和b列的行数据相等才会合并)
2.分组筛选(having\聚合函数)
3.对各分组数据进行统计(聚合函数)

order by

order by a,b (先按a排序再按b排序)