通过SQL Server中的递归CTE创建排列？

Question

看着 :

;WITH cte AS(
    SELECT 1 AS x UNION
    SELECT 2 AS x UNION     
    SELECT 3 AS x   
)

我可以为所有3个值创建排列表:

SELECT T1.x , y=T2.x , z=t3.x
FROM cte T1
JOIN cte T2
ON T1.x != T2.x
JOIN cte T3
ON T2.x != T3.x AND T1.x != T3.x

这使用了SQL笛卡尔积的功效,并消除了相等的值.

好.

但是有可能增强这种递归伪CTE:

;WITH cte AS(
    SELECT 1 AS x ,  2 AS y , 3 AS z   
    UNION ALL 
    ...
)

SELECT * FROM cte

因此它将产生与以下相同的结果:

注意,在SO中有其他使用递归CTE的解决方案,但它没有传播到列,而是排列的字符串表示

Answer 1

我尝试在 CTE 中完成很多工作。

然而，尝试动态“重新定义”行集有点棘手。虽然使用动态 SQL 来完成该任务相对容易，但不会带来任何问题。

虽然这个答案可能不是最有效或最直接的，甚至在它不完全是 CTE 的意义上是正确的，但它可以为其他人提供工作的基础。

为了更好地理解我的方法，请阅读注释，但可能值得通过更改主块中下面的代码位来依次查看每个 CTE 表达式，并注释掉下面的部分。

SELECT * FROM <CTE NAME>

祝你好运。

IF OBJECT_ID('tempdb..#cteSchema') IS NOT NULL
    DROP Table #cteSchema
GO

-- BASE CTE
;WITH cte AS( SELECT 1 AS x, 2 AS y, 3 AS z),

    -- So we know what columns we have from the CTE we extract it to XML
    Xml_Schema AS ( SELECT CONVERT(XML,(SELECT * FROM cte FOR XML PATH(''))) AS MySchema ),

    -- Next we need to get a list of the columns from the CTE, by querying the XML, getting the values and assigning a num to the column
    MyColumns AS (SELECT D.ROWS.value('fn:local-name(.)','SYSNAME') AS ColumnName,
                        D.ROWS.value('.','SYSNAME') as Value,
                        ROW_NUMBER() OVER (ORDER BY D.ROWS.value('fn:local-name(.)','SYSNAME')) AS Num
                    FROM Xml_Schema
                        CROSS APPLY Xml_Schema.MySchema.nodes('/*') AS D(ROWS) ),
    -- How many columns we have in the CTE, used a coupld of times below
    ColumnStats AS (SELECT MAX(NUM) AS ColumnCount FROM MyColumns),

    -- create a cartesian product of the column names and values, so now we get each column with it's possible values,
    -- so {x=1, x =2, x=3, y=1, y=2, y=3, z=1, z=2, z=3} -- you get the idea.
    PossibleValues AS (SELECT MyC.ColumnName, MyC.Num AS ColumnNum, MyColumns.Value, MyColumns.Num, 
                ROW_NUMBER() OVER (ORDER BY MyC.ColumnName, MyColumns.Value, MyColumns.Num ) AS ID
                FROM MyColumns 
                    CROSS APPLY MyColumns MyC
                ),

    -- Now we have the possibly values of each "column" we now have to concat the values together using this recursive CTE.
    AllRawXmlRows AS (SELECT CONVERT(VARCHAR(MAX),'<'+ISNULL((SELECT ColumnName FROM MyColumns WHERE MyColumns.Num = 1),'')+'>'+Value) as ConcatedValue, Value,ID, Counterer = 1  FROM PossibleValues
                UNION ALL
            SELECT CONVERT(VARCHAR(MAX),CONVERT(VARCHAR(MAX), AllRawXmlRows.ConcatedValue)+'</'+(SELECT ColumnName FROM MyColumns WHERE MyColumns.Num = Counterer)+'><'+(SELECT ColumnName FROM MyColumns WHERE MyColumns.Num = Counterer+1)+'>'+CONVERT(VARCHAR(MAX),PossibleValues.Value))  AS ConcatedValue, PossibleValues.Value, PossibleValues.ID,
            Counterer = Counterer+1
            FROM AllRawXmlRows
                INNER JOIN PossibleValues  ON AllRawXmlRows.ConcatedValue NOT LIKE '%'+PossibleValues.Value+'%' -- I hate this, there has to be a better way of making sure we don't duplicate values.... 
                AND AllRawXmlRows.ID <> PossibleValues.ID
                AND Counterer < (SELECT ColumnStats.ColumnCount FROM ColumnStats)
                ),

    -- The above made a list but was missing the final closing XML element. so we add it.
    -- we also restict the list to the items that contain all columns, the section above builds it up over many columns
    XmlRows AS (SELECT DISTINCT 
                ConcatedValue +'</'+(SELECT ColumnName FROM MyColumns WHERE MyColumns.Num = Counterer)+'>' 
                AS ConcatedValue
            FROM AllRawXmlRows WHERE Counterer = (SELECT ColumnStats.ColumnCount FROM ColumnStats)
                    ),              
    -- Wrap the output in row and table tags to create the final XML
    FinalXML AS (SELECT (SELECT CONVERT(XML,(SELECT CONVERT(XML,ConcatedValue) FROM XmlRows FOR XML PATH('row'))) FOR XML PATH('table') )as XMLData),

    -- Prepare a CTE that represents the structure of the original CTE with 
    DataTable AS (SELECT cte.*, XmlData
                        FROM FinalXML, cte)
--SELECT * FROM <CTE NAME>
    -- GETS destination columns with XML data.
SELECT * 
    INTO #cteSchema
FROM DataTable


DECLARE @XML VARCHAR(MAX) ='';
SELECT @Xml = XMLData FROM #cteSchema --Extract XML Data from the

ALTER TABLE #cteSchema DROP Column XMLData -- Removes the superflous column
DECLARE @h INT
EXECUTE sp_xml_preparedocument @h OUTPUT, @XML
    SELECT * 
        FROM OPENXML(@h, '/table/row', 2) 
            WITH #cteSchema -- just use the #cteSchema to define the structure of the xml that has been constructed

EXECUTE sp_xml_removedocument @h