SQL 中的中位值计算

1. 在Northwind数据库上建立一个视图 VOrders

代码

SET  NOCOUNT  ON ;
USE  Northwind;
GO

IF   OBJECT_ID ( ' dbo.VOrders ' )  IS   NOT   NULL
   DROP   VIEW  dbo.VOrders;
GO
CREATE   VIEW  dbo.VOrders
AS

SELECT  O.OrderID, O.OrderDate, O.CustomerID, O.EmployeeID, O.ShipVia,
   SUM (OD.Quantity)  AS  Qty,
   CAST ( SUM (OD.Quantity  *  UnitPrice  *  ( 1   -  Discount))  AS   DECIMAL ( 12 ,  2 ))  AS  Value 
FROM  dbo.Orders  AS  O
   JOIN   [ Order Details ]   AS  OD
     ON  O.OrderID  =  OD.OrderID
GROUP   BY  O.OrderID, O.OrderDate, O.CustomerID, O.EmployeeID, O.ShipVia
GO

 
2. 计算整个表中数据的中位值 （MSSQL 2000的做法）

代码

SELECT
(
 ( SELECT   MAX (Value)  FROM
   ( SELECT   TOP   50   PERCENT  Value  FROM  dbo.VOrders  ORDER   BY  Value)  AS  H1)
  +
 ( SELECT   MIN (Value)  FROM
   ( SELECT   TOP   50   PERCENT  Value  FROM  dbo.VOrders  ORDER   BY  Value  DESC )  AS  H2)
)  /   2   AS  Median;

 

3. 用子查询来查询每个员工的中位值（MSSQL 2000的做法）

代码

SELECT  EmployeeID,
(
 ( SELECT   MAX (Value)  FROM
   ( SELECT   TOP   50   PERCENT  Value  FROM  dbo.VOrders  AS  O1
     WHERE  O1.EmployeeID  =  E.EmployeeID
     ORDER   BY  Value)  AS  H1)
  +
 ( SELECT   MIN (Value)  FROM
   ( SELECT   TOP   50   PERCENT  Value  FROM  dbo.VOrders  AS  O2
     WHERE  O2.EmployeeID  =  E.EmployeeID
     ORDER   BY  Value  DESC )  AS  H2)
)  /   2   AS  Median
FROM  dbo.Employees  AS  E;

 

4.1 SQL 2005的做法，首先我们看看用来计算中位值的行的数据，这里用到了通用表（CTE）和Row_Number函数

代码

WITH  OrdersRN  AS
(
   SELECT  EmployeeID, Value,
    ROW_NUMBER()  OVER (PARTITION  BY  EmployeeID  ORDER   BY  Value)  AS  RowNum,
     COUNT ( * )  OVER (PARTITION  BY  EmployeeID)  AS  Cnt
   FROM  dbo.VOrders
)
SELECT  EmployeeID, Value, RowNum, Cnt
FROM  OrdersRN
WHERE  RowNum  IN ((Cnt  +   1 )  /   2 , (Cnt  +   2 )  /   2 );

 

4.2 SQL 2005中，用通用表和Row_Number函数来计算中位值

 

代码

WITH  OrdersRN  AS
(
   SELECT  EmployeeID, Value,
    ROW_NUMBER()  OVER (PARTITION  BY  EmployeeID  ORDER   BY  Value)  AS  RowNum,
     COUNT ( * )  OVER (PARTITION  BY  EmployeeID)  AS  Cnt
   FROM  dbo.VOrders
)
SELECT  EmployeeID,  AVG (Value)  AS  Median
FROM  OrdersRN
WHERE  RowNum  IN ((Cnt  +   1 )  /   2 , (Cnt  +   2 )  /   2 )
GROUP   BY  EmployeeID;

 

 

关键词：中位值，中位值计算，Median，CTE，Row_Number