我要评论一、前言
关于同表分组，今天发现partition by 特好用，
相比于group by,贵在保留原表所有行+灵活计算
二、案例
对于同表tablea 的不同分组计数，如:
select *, (select count(1) tablea b where a.col1=b.col1) as count1 (select count(1) tablea b where a.col1=b.col1 and a.col2=b.col2 ) as count2 (select count(1) tablea b where a.col1=b.col1 and a.col2=b.col2 and a.col3=b.col3) as count3 from tablea a
使用partition by：
select *, count(1) over (partition by col1) as count1 count(1) over (partition by col1,col2) as count2 count(1) over (partition by col1,col2,col3) as count3 from tablea a
代码一下清爽了。
三、与group by 对比
实现计数
select *, count(1) over (partition by col1) as count1 from tablea a
相当于
select *, (select count(1) tablea b where a.col1=b.col1 group by col1) as count1 from tablea a
四、妙用
4.1 接函数
select
*,
-- 计算总销
sum(amount) over (partition by col1 ) as total,
-- 计算占比（保留2位小数）
round(col3 / sum(col2 ) over (partition by col1 ) * 100, 2) as ratio
from tablea ;
4.2 行号
select
*,
row_number() over (partition by col1 order by col2 desc)
from tablea ;
多一个行号列 1、2、3、4、5
4.3 count 另类
count 会遍历所有数据， 但是row_number()-1 只会取一条，
这未曾不是一种优化
select
*,
row_number() over (partition by col1 order by col2 desc) -1 as count1
from tablea ;
--备注
select
*,
case when count1=0 then 1 else 2 end as '新产品'
from tablea ;
当然 还可以 max(count1)-1 作为 总条数的取值。
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