CUBE是对GROUP BY子句的另外一 种扩充，它允许计算所有小计可能的组合，用来生成各种汇总报表。
以下介绍他们的用法
select B,c,D from Atest group by rollup(B,C,D);
相当于
select B,c,D from Atest group by B,C,D;
union all
select B,c,null from Atest group by B,C
union all
select B,null,null from Atest group by B
select B,c from Atest group by cube(B,C);
将所有 (B,C)组合的子集group by合并
相当于
select B,c from Atest group by B,C
union all
select B,null from Atest group by B
union all
select null,c from Atest group by c
union all
select null,null from Atest group by null