T-SQL Part IX, PIVOT and UNPIVOT

不同於CROSS JOIN, CROSS APPLY, OUTER APPLY，MSDN文檔對PIVOT和UNPIVOT 想得重視了一點，單獨做了一個頁面來介紹。
簡單來說，PIVOT用來把行轉成列，而UNPIVOT可以把列轉成行。

用MSDN文檔給出的兩個例子來做說明。
例一，基礎示例。

-- Pivot table with one row and five columns  
SELECT 'AverageCost' AS Cost_Sorted_By_Production_Days,   
[0], [1], [2], [3], [4]  
FROM  
(SELECT DaysToManufacture, StandardCost   
    FROM Production.Product) AS SourceTable  
PIVOT  
(  
AVG(StandardCost)  
FOR DaysToManufacture IN ([0], [1], [2], [3], [4])  
) AS PivotTable;  

例二，複雜示例。

USE AdventureWorks2014;  
GO  
SELECT VendorID, [250] AS Emp1, [251] AS Emp2, [256] AS Emp3, [257] AS Emp4, [260] AS Emp5  
FROM   
(SELECT PurchaseOrderID, EmployeeID, VendorID  
FROM Purchasing.PurchaseOrderHeader) p  
PIVOT  
(  
COUNT (PurchaseOrderID)  
FOR EmployeeID IN  
( [250], [251], [256], [257], [260] )  
) AS pvt  
ORDER BY pvt.VendorID;  

第一步，也是所有SELECT語句的第一步，弄清楚Source Table，即FROM後面的源table。
對於例一：

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

對於例二：

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

第二步，理解PIVOT的作用域。
對於例一，即針對DaysToManufacture在IN範疇中的每個值計算AVG(StandardCost)

(  
AVG(StandardCost)  
FOR DaysToManufacture IN ([0], [1], [2], [3], [4])  
)

對於例二，即針對EmployID在IN範疇中的每一個值計算COUNT (PurchaseOrderID)：

(  
COUNT (PurchaseOrderID)  
FOR EmployeeID IN  
( [250], [251], [256], [257], [260] )  
)

事實上，可以用等價的SQL來實現PIVOT。以下兩段SQL是等價的：

SELECT empid, [2013], [2014], [2015]
FROM ( SELECT empid, YEAR(orderdate) AS orderyear, val
       FROM Sales.OrderValues                          ) AS D
  PIVOT( SUM(val) FOR orderyear IN([2013],[2014],[2015]) ) AS P;

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

UNPIVOT執行的是PIVOT相反但原理完全一致的操作。

然而，有兩個問題比較困擾我：

1. 如何執行動態的PIVOT？比如，示例中的IN（）部分都是寫死的，現實中顯然這種固定值的情況不多。搜索了一圈下來，答案几乎全都是手動拼接IN後面的字符串。有一篇Blog值得一看。
2. 這個功能在現實中的意義？貌似這個沒有標準答案。無論如何，行列互轉總是個噱頭。

是爲之記。
Alva Chien
2016.6.14