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：

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。
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

巴特西

T-SQL Part IX, PIVOT and UNPIVOT

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