Question

假设此表：

PruchaseID | Customer | Product  | Method
-----------|----------|----------|--------
 1         | John     | Computer | Credit
 2         | John     | Mouse    | Cash
 3         | Will     | Computer | Credit
 4         | Will     | Mouse    | Cash
 5         | Will     | Speaker  | Cash
 6         | Todd     | Computer | Credit

我希望为每个客户生成他们购买的产品及其付款方式的报告但我希望每个客户的报告都是一行，例如：

Customer | Products                 | Methods
---------|--------------------------|--------------
 John    | Computer, Mouse          | Credit, Cash
 Will    | Computer, Mouse, Speaker | Credit, Cash
 Todd    | Computer                 | Credit

到目前为止我发现的是使用XML PATH方法进行分组连接，例如：

SELECT
    p.Customer,
    STUFF(
        SELECT ', ' + xp.Product
        FROM Purchases xp
        WHERE xp.Customer = p.Customer
        FOR XML PATH('')), 1, 1, '') AS Products,
    STUFF(
        SELECT ', ' + xp.Method
        FROM Purchases xp
        WHERE xp.Customer = p.Customer
        FOR XML PATH('')), 1, 1, '') AS Methods
FROM Purchases

这给了我结果，但我关心的是这个速度乍一看，这里有三个不同的选择，两个将乘以购买的行数。最终这会在一般情况下放慢速度。

那么，有没有办法以更好的表现来做到这一点？我想添加更多的列来聚合，我应该为每一列做这个STUFF（）块吗？这对我来说听起来不够快。

Siggestions？

Answer 1

只是一个想法：

DECLARE @t TABLE (
    Customer VARCHAR(50),
    Product VARCHAR(50),
    Method VARCHAR(50),
    INDEX ix CLUSTERED (Customer)
)

INSERT INTO @t (Customer, Product, Method)
VALUES
    ('John', 'Computer', 'Credit'),
    ('John', 'Mouse', 'Cash'),
    ('Will', 'Computer', 'Credit'),
    ('Will', 'Mouse', 'Cash'),
    ('Will', 'Speaker', 'Cash'),
    ('Todd', 'Computer', 'Credit')

SELECT t.Customer
     , STUFF(CAST(x.query('a/text()') AS NVARCHAR(MAX)), 1, 2, '')
     , STUFF(CAST(x.query('b/text()') AS NVARCHAR(MAX)), 1, 2, '')
FROM (
    SELECT DISTINCT Customer
    FROM @t
) t
OUTER APPLY (
    SELECT DISTINCT [a] = CASE WHEN id = 'a' THEN ', ' + val END
                  , [b] = CASE WHEN id = 'b' THEN ', ' + val END
    FROM @t t2
    CROSS APPLY (
        VALUES ('a', t2.Product)
             , ('b', t2.Method)
    ) t3 (id, val)
    WHERE t2.Customer = t.Customer
    FOR XML PATH(''), TYPE
) t2 (x)

输出：

Customer   Product                    Method     
---------- -------------------------- ------------------
John       Computer, Mouse            Cash, Credit
Todd       Computer                   Credit
Will       Computer, Mouse, Speaker   Cash, Credit

另一个具有更多性能优势的想法：

IF OBJECT_ID('tempdb.dbo.#EntityValues') IS NOT NULL
    DROP TABLE #EntityValues

DECLARE @Values1 VARCHAR(MAX)
      , @Values2 VARCHAR(MAX)

SELECT Customer
     , Product
     , Method
     , RowNum = ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY 1/0)
     , Values1 = CAST(NULL AS VARCHAR(MAX))
     , Values2 = CAST(NULL AS VARCHAR(MAX))
INTO #EntityValues
FROM @t

UPDATE #EntityValues
SET 
      @Values1 = Values1 =
        CASE WHEN RowNum = 1 
            THEN Product
            ELSE @Values1 + ', ' + Product 
        END
    , @Values2 = Values2 = 
        CASE WHEN RowNum = 1 
            THEN Method
            ELSE @Values2 + ', ' + Method
        END

SELECT Customer
      , Values1 = MAX(Values1) 
      , Values2 = MAX(Values2)
FROM #EntityValues
GROUP BY Customer

但有一些限制：

Customer      Values1                       Values2
------------- ----------------------------- ----------------------
John          Computer, Mouse               Credit, Cash
Todd          Computer                      Credit
Will          Computer, Mouse, Speaker      Credit, Cash, Cash

同时查看关于字符串聚合的旧帖子：

http://www.codeproject.com/Articles/691102/String-Aggregation-in-the-World-of-SQL-Server

Answer 2

这是递归CTE（公用表表达式）的用例之一。您可以在https://technet.microsoft.com/en-us/library/ms190766(v=sql.105).aspx

了解更多信息

;
WITH CTE1 (PurchaseID, Customer, Product, Method, RowID)
AS
(
    SELECT 
        PurchaseID, Customer, Product, Method, 
        ROW_NUMBER() OVER (PARTITION BY Customer ORDER BY Customer)
    FROM
        @tbl 
        /* This table holds source data. I ommited declaring and inserting 
        data into it because that's not important. */
)
, CTE2 (PurchaseID, Customer, Product, Method, RowID)
AS
(
    SELECT 
        PurchaseID, Customer, 
        CONVERT(VARCHAR(MAX), Product), 
        CONVERT(VARCHAR(MAX), Method), 
        1
    FROM 
        CTE1 
    WHERE 
        RowID = 1
    UNION ALL
    SELECT 
        CTE2.PurchaseID, CTE2.Customer, 
        CONVERT(VARCHAR(MAX), CTE2.Product + ',' + CTE1.Product), 
        CONVERT(VARCHAR(MAX), CTE2.Method + ',' + CTE1.Method), 
        CTE2.RowID + 1 
    FROM 
        CTE2 INNER JOIN CTE1 
            ON CTE2.Customer = CTE1.Customer
            AND CTE2.RowID + 1 = CTE1.RowID
)

SELECT Customer, MAX(Product) AS Products, MAX(Method) AS Methods 
FROM CTE2 
GROUP BY Customer

输出：

Customer    Products                Methods
John        Computer,Mouse          Credit,Cash
Todd        Computer                Credit
Will        Computer,Mouse,Speaker  Credit,Cash,Cash

Answer 3

另一个解决方案是用于组连接的CLR方法@aaron bertrand已对此here进行了性能比较。如果您可以部署CLR，则从http://groupconcat.codeplex.com/下载免费的脚本。所有细节都在文档中。您的查询将改为像这样

SELECT Customer,dbo.GROUP_CONCAT(product),dbo.GROUP_CONCAT(method)
FROM Purchases
GROUP BY Customer

这个查询很简单，易于记忆和使用，XML方法也可以完成这项工作，但是记住代码有点困难（至少对我来说）并且像XML一样可以解决问题，这可以解决问题，也可以解决一些陷阱在他的博客中描述。

同样从性能角度来看，使用.query很耗时我遇到了同样的性能问题。我希望你能在https://dba.stackexchange.com/questions/125771/multiple-column-concatenation中找到我提出的这个问题。检查kenneth fisher给出的第2版嵌套xml连接方法或spaggettidba建议的unpivot / pivot方法。

如何分组连接多个列？

3 个答案: