如何使用PIVOT在Select查询中动态声明列

Question

我正在编写查询以获取PersonID的地址。以下查询对我有用，但它只返回两个地址。我想用单个查询处理'n'个地址。有没有办法做到这一点？

非常感谢

SELECT 
  PersonID, PersonName
  [Address1], [Address2]
FROM
  (
  SELECT 
    P.PersonID, 
    P.PersonName, 
    (ROW_NUMBER() OVER(PARTITION BY P.PersonID ORDER BY A.AddressID)) RowID
    FROM tblPerson
    INNER JOIN tblAddress AS A ON A.PersonID = P.PersonID
  ) AS AddressTable
PIVOT
  (
    MAX(AddressID)
    FOR RowID IN ([Address1], [Address2])
  ) AS PivotTable;

Answer 1

假设以下表格和样本数据：

USE tempdb;
GO

CREATE TABLE dbo.tblPerson(PersonID INT, PersonName VARCHAR(255));

INSERT dbo.tblPerson SELECT 1, 'Bob'
           UNION ALL SELECT 2, 'Charlie'
           UNION ALL SELECT 3, 'Frank'
           UNION ALL SELECT 4, 'Amore';

CREATE TABLE dbo.tblAddress(AddressID INT, PersonID INT, [Address] VARCHAR(255));

INSERT dbo.tblAddress SELECT 1,1,'255 1st Street'
            UNION ALL SELECT 2,2,'99 Elm Street'
            UNION ALL SELECT 3,2,'67 Poplar Street'
            UNION ALL SELECT 4,2,'222 Oak Ave.'
            UNION ALL SELECT 5,1,'36 Main Street, Suite 22'
            UNION ALL SELECT 6,4,'77 Sicamore Ct.';

以下查询获取所需的结果，并显示它如何处理0,1或n个地址。在这种情况下，最高数字为3，但如果您愿意，可以通过稍微调整样本数据来播放更多地址。

DECLARE @col   NVARCHAR(MAX) = N'',
        @sel   NVARCHAR(MAX) = N'',
        @from  NVARCHAR(MAX) = N'',
        @query NVARCHAR(MAX) = N'';

;WITH m(c) AS
(
    SELECT TOP 1 c = COUNT(*)
    FROM dbo.tblAddress
    GROUP BY PersonID
    ORDER BY c DESC
)
SELECT @col = @col + ',[Address' + RTRIM(n.n) + ']',
    @sel = @sel + ',' + CHAR(13) + CHAR(10) + '[Address' + RTRIM(n.n) + '] = x' 
        + RTRIM(n.n) + '.Address',
    @from = @from + CHAR(13) + CHAR(10) + ' LEFT OUTER JOIN xMaster AS x' 
        + RTRIM(n.n) + ' ON x' + RTRIM(n.n) + '.PersonID = p.PersonID AND x' 
        + RTRIM(n.n) + '.rn = ' + RTRIM(n.n)
FROM m CROSS JOIN (SELECT n = ROW_NUMBER() OVER (ORDER BY [object_id]) 
    FROM sys.all_columns) AS n WHERE n.n <= m.c;

SET @query = N';WITH xMaster AS 
(
  SELECT PersonID, Address, 
    rn = ROW_NUMBER() OVER (PARTITION BY PersonID ORDER BY Address) 
    FROM dbo.tblAddress
)
SELECT PersonID, PersonName' + @col
+ ' FROM
    (
        SELECT p.PersonID, p.PersonName, ' + STUFF(@sel, 1, 1, '')
 + CHAR(13) + CHAR(10) + ' FROM dbo.tblPerson AS p ' + @from + '
 ) AS Addresses;';

PRINT @query;
--EXEC sp_executesql @query;

如果您打印SQL，您将看到以下结果：

;WITH xMaster AS 
(
  SELECT PersonID, Address, 
    rn = ROW_NUMBER() OVER (PARTITION BY PersonID ORDER BY Address) 
    FROM dbo.tblAddress
)
SELECT PersonID, PersonName,[Address1],[Address2],[Address3] FROM
    (
        SELECT p.PersonID, p.PersonName, 
 [Address1] = x1.Address,
 [Address2] = x2.Address,
 [Address3] = x3.Address 
FROM dbo.tblPerson AS p 
 LEFT OUTER JOIN xMaster AS x1 ON x1.PersonID = p.PersonID AND x1.rn = 1
 LEFT OUTER JOIN xMaster AS x2 ON x2.PersonID = p.PersonID AND x2.rn = 2
 LEFT OUTER JOIN xMaster AS x3 ON x3.PersonID = p.PersonID AND x3.rn = 3
 ) AS Addresses;

如果你执行它，你会看到：

我知道要到达这里的查询是一个丑陋的混乱，但你的要求决定了它。如我在评论中建议的那样返回逗号分隔列表会更容易，或者让表示层处理旋转。