逆透视转换

逆透视转换是一种将数据从列的状态转换成行的状态的一种技术。进行逆透视转换一般要经历三个逻辑处理阶段：生成副本，提取元素和删除不相关的交叉。

下面是一个进行逆透视转换的示例。

USE tempdb;

GO



-- 逆透视转换(列转行)

-- 准备测试数据

IF OBJECT_ID('dbo.Orders','U') IS NOT NULL DROP TABLE dbo.Orders;

GO



CREATE TABLE dbo.Orders

(

    orderid INT NOT NULL,

    orderdate DATETIME NOT NULL,

    empid INT NOT NULL,

    custid VARCHAR(5) NOT NULL,

    qty INT NOT NULL,

    CONSTRAINT PK_Orders PRIMARY KEY(orderid)

);



INSERT INTO dbo.Orders(orderid,orderdate,empid,custid,qty) 

VALUES 

    (30001,'20070802',3,'A',10),

    (10001,'20071224',2,'A',12),

    (10005,'20071224',1,'B',20),

    (40001,'20080109',2,'A',40),

    (10006,'20080118',1,'C',14),

    (20001,'20080212',2,'B',12),

    (40005,'20090212',3,'A',10),

    (20002,'20090216',1,'C',20),

    (30003,'20090418',2,'B',15),

    (30004,'20070418',3,'C',22),

    (30007,'20090907',3,'D',30);

    

IF OBJECT_ID('dbo.EmpCustOrders','U') IS NOT NULL DROP TABLE dbo.EmpCustOrders;

GO



SELECT empid,A,B,C,D

INTO dbo.EmpCustOrders

FROM (SELECT empid,custid,qty FROM dbo.Orders) AS D

PIVOT(SUM(D.qty) FOR D.custid IN (A,B,C,D)) AS P;



SELECT * FROM dbo.EmpCustOrders;

逆透视前：

逆透视也有两种解决方案，一是使用标准SQL，另一种是使用UNPIVOT运算符。下面是示例代码。

-- 1,使用标准SQL

-- 交叉联接，派生表

SELECT * 

FROM (SELECT empid,custid,

        CASE custid

            WHEN 'A' THEN A

            WHEN 'B' THEN B

            WHEN 'C' THEN C

            WHEN 'D' THEN D

        END AS qty 

        FROM dbo.EmpCustOrders

            CROSS JOIN (SELECT 'A' AS custid

                        UNION ALL SELECT 'B'

                        UNION ALL SELECT 'C'

                        UNION ALL SELECT 'D') AS Custs) AS D

WHERE qty IS NOT NULL;



-- 2,使用UNPIVOT运算符

SELECT empid,custid,qty 

FROM dbo.EmpCustOrders

UNPIVOT (qty FOR custid IN (A,B,C,D)) AS U;

逆透视后的效果如下图。