SQL之透视、逆透视及分组集

透视

假如当前有数据源如下所示：

有一报表需求如下所示：

这一类的需求就称之为数据透视转换。透视转换一般涉及分组、扩展及聚合三个阶段。上面的需求是按照empid进行分组，按照custid对订货量进行扩展，最后进行聚合SUM(qty)。数据透视转换其实是存在某种通用查询模板的。下面是上面这个需求的标准SQL解决方案：

SELECT empid,
  SUM(CASE WHEN custid = 'A' THEN qty END) AS A,
  SUM(CASE WHEN custid = 'B' THEN qty END) AS B,
  SUM(CASE WHEN custid = 'C' THEN qty END) AS C,
  SUM(CASE WHEN custid = 'D' THEN qty END) AS D  
FROM dbo.Orders
GROUP BY empid;

下面是使用T-SQL PIVOT运算符进行透视转换：

SELECT empid, A, B, C, D
FROM (SELECT empid, custid, qty
      FROM dbo.Orders) AS D
  PIVOT(SUM(qty) FOR custid IN(A, B, C, D)) AS P;

注意，一般不直接把PIVOT运算符应用到源数据表(本例为Orders表)，而是将其应用到一个表表达式(该表表达式只包含透视转换需要的3种元素，不包含其他属性)。

逆透视

标准SQL的解决方案：

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 (VALUES('A'),('B'),('C'),('D')) AS Custs(custid)) AS D
WHERE qty IS NOT NULL;

使用T-SQL的UNPIVOT运算符进行逆透视转换：

SELECT empid, custid, qty
FROM dbo.EmpCustOrders
  UNPIVOT(qty FOR custid IN(A, B, C, D)) AS U;