T-SQL Part IX, PIVOT and UNPIVOT

不同于CROSS JOIN, CROSS APPLY, OUTER APPLY，MSDN文档对PIVOT和UNPIVOT 想得重视了一点，单独做了一个页面来介绍。
简单来说，PIVOT用来把行转成列，而UNPIVOT可以把列转成行。

用MSDN文档给出的两个例子来做说明。
例一，基础示例。

-- Pivot table with one row and five columns
SELECT 'AverageCost' AS Cost_Sorted_By_Production_Days,
[], [], [], [], []
FROM
(SELECT DaysToManufacture, StandardCost
    FROM Production.Product) AS SourceTable
PIVOT
(
AVG(StandardCost)
FOR DaysToManufacture IN ([], [], [], [], [])
) AS PivotTable;  

例二，复杂示例。

USE AdventureWorks2014;
GO
SELECT VendorID, [] AS Emp1, [] AS Emp2, [] AS Emp3, [] AS Emp4, [] AS Emp5
FROM
(SELECT PurchaseOrderID, EmployeeID, VendorID
FROM Purchasing.PurchaseOrderHeader) p
PIVOT
(
COUNT (PurchaseOrderID)
FOR EmployeeID IN
( [], [], [], [], [] )
) AS pvt
ORDER BY pvt.VendorID;  

第一步，也是所有SELECT语句的第一步，弄清楚Source Table，即FROM后面的源table。
对于例一：

(SELECT DaysToManufacture, StandardCost
    FROM Production.Product) AS SourceTable 

对于例二：

(SELECT PurchaseOrderID, EmployeeID, VendorID
FROM Purchasing.PurchaseOrderHeader) p

第二步，理解PIVOT的作用域。
对于例一，即针对DaysToManufacture在IN范畴中的每个值计算AVG(StandardCost)

(
AVG(StandardCost)
FOR DaysToManufacture IN ([], [], [], [], [])
)

对于例二，即针对EmployID在IN范畴中的每一个值计算COUNT (PurchaseOrderID)：

(
COUNT (PurchaseOrderID)
FOR EmployeeID IN
( [], [], [], [], [] )
)

事实上，可以用等价的SQL来实现PIVOT。以下两段SQL是等价的：

SELECT empid, [], [], []
FROM ( SELECT empid, YEAR(orderdate) AS orderyear, val
       FROM Sales.OrderValues                          ) AS D
  PIVOT( SUM(val) FOR orderyear IN([],[],[]) ) AS P;
SELECT empid,
  SUM(CASE WHEN orderyear = 2013 THEN val END) AS [],
  SUM(CASE WHEN orderyear = 2014 THEN val END) AS [],
  SUM(CASE WHEN orderyear = 2015 THEN val END) AS []
FROM ( SELECT empid, YEAR(orderdate) AS orderyear, val
       FROM Sales.OrderValues                          ) AS D
GROUP BY empid;

UNPIVOT执行的是PIVOT相反但原理完全一致的操作。

然而，有两个问题比较困扰我：

1. 如何执行动态的PIVOT？比如，示例中的IN（）部分都是写死的，现实中显然这种固定值的情况不多。搜索了一圈下来，答案几乎全都是手动拼接IN后面的字符串。有一篇Blog值得一看。
2. 这个功能在现实中的意义？貌似这个没有标准答案。无论如何，行列互转总是个噱头。

是为之记。
Alva Chien
2016.6.14