SQL PARTITION BY用法

文章目录

一、前言
二、案例
[三、与group by 对比](#三、与group by 对比)
四、妙用
- [4.1 接函数](#4.1 接函数)
- [4.2 行号](#4.2 行号)
- [4.3 count 另类](#4.3 count 另类)

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:

sql 复制代码

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：

sql 复制代码

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

sql 复制代码

SELECT *, 
count(1) OVER  (PARTITION BY COL1) AS COUNT1
FROM TableA A

相当于

sql 复制代码

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

sql 复制代码

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

sql 复制代码

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

sql 复制代码

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。