继续学习窗口函数Over

首先, 我们确定Sql语句执行大致流程

• FROM   =>> WHERE =>> GROUP BY =>> HAVING =>> SELECT ==>

•             { Over

•                Distinct

•                Top} ==>

• ORDER BY

其中比较难一点的就是Over窗口函数了.

Over单独配合聚合函数

SELECT * FROM Sales.OrderValues;

有表如图1

SELECT orderid, custid, val,orderdate
  ,SUM(val) OVER() AS totalvalue,
  AVG(val) over() as avgvalue
FROM Sales.OrderValues;

图2

对比上图, 发现除了增加了两列外, 没有改变排序.

SELECT orderid, custid, val,orderdate
  ,SUM(val) OVER() AS totalvalue,
  AVG(val) over() as avgvalue,
  SUM(val) OVER(PARTITION BY custid) AS custtotalvalue
  /*,AVG(val) over(PARTITION BY orderdate) as custAvgValue*/
FROM Sales.OrderValues;
图3

对比图1. 发现, Partition By 不仅让sum聚合函数只计算CustID区域的val. 并且使得结果按custid排序

SELECT orderid, custid, val,orderdate
  ,SUM(val) OVER() AS totalvalue,
  AVG(val) over() as avgvalue
  ,SUM(val) OVER(PARTITION BY custid) AS custtotalvalue
  ,AVG(val) over(PARTITION BY orderdate) as custAvgValue
FROM Sales.OrderValues;
图4
对比图3, 发现如果再增加一个有partitionby子句的窗口函数, 结果按照后一个(这里是orderdate)排序

SELECT orderid, custid, val,orderdate,ROW_NUMBER() over(order by val desc)
  ,SUM(val) OVER() AS totalvalue,
  AVG(val) over() as avgvalue
  ,SUM(val) OVER(PARTITION BY custid) AS custtotalvalue
  /*,AVG(val) over(PARTITION BY orderdate) as custAvgValue*/
  FROM Sales.OrderValues;

结果同图4一样. 虽然增加了一列RowIndex列, 但是结果依然是按照custid排序, 除非将那一句放到后面.

如果排序函数中使用了partition by子句. 那么, 返回的结果还是以partition by子句的字段升序.

 

当然, 以上结果是在没有order by排序的前提下. 如果最外层有order by, 那么还是按order by 排序.