• Pivot 和 Unpviot的概念，把列塞入行，和把行塞入列
• Unnest的概念，把tuple转换成行
• 造数据并且自测

with (
Select 10 as count, "a" as name union all 
Select 20, "b" union all
select 30, "c"
)
as test
select * from test

• cross join就是纯笛卡尔积，cross join + where a.id = b.id <=> inner join
• group by rollup 是BI中经常用的概念， group by rollup(a,b) 等同于把 group by a,b, group by a 和group by ()三个结合在一起，同时返回三个维度的数据