公用表表达式检索路径

Question

我试图使用递归查询通过结构化像这样的表找到一个路径：

RelatedEntities

FromKey TINYINT 
ToKey TINYINT 
...more.... 

我想我可以做类似这：

DECLARE @startKey UNIQUEIDENTIFIER, @endKey UNIQUEIDENTIFIER; 
SET @startKey = 0; 
SET @endKey = 3; 

;With findPath 
AS 
(
    SELECT FromKey, ToKey 
    FROM RelatedEntities 
    WHERE FromKey = @startKey 

    UNION ALL 

    SELECT FromKey, ToKey 
    FROM RelatedEntities r 
    JOIN findPath b 
    ON r.FromKey = b.ToKey 
    AND r.FromKey NOT IN (SELECT FromKey FROM b) 
) 
SELECT * FROM findPath; 

此代码失败，因为我不能使用CTE内的子查询。它似乎也是一个规则，递归查询只能包含一个引用到CTE。 （true？）也许这是一个游标或程序代码的工作，但我想我会把它放在这里以防万一我找不到通过CTE表的路径的方法？

的参数是：

1. 开始有开始和结束键
2. 基地查询使用开始键
3. 递归查询时，它所包含的结束键应该停止，（一直没能指出一个），并且不应该重复开始键。
4. MAXRECURSION选项可用于在一定次数的迭代后停止。

感谢你们所有的CTE专家。把我放直。

为了便于阅读，将其从UNIQUEIDENTIFIERS更改为TINYINT。 SQL结构是相同的。这里有一些代码来测试它。

CREATE TABLE RelatedEntities(FromKey TINYINT, ToKey TINYINT); 

INSERT RelatedEntities(FromKey, ToKey) 
VALUES 
(1, 0), 
(0, 1), 
(1, 7), 
(7, 1), 
(3, 4), 
(4, 3) 

;With FindPath 
AS 
(
    SELECT FromKey, ToKey, 0 AS recursionLevel 
    FROM RelatedEntities 
    WHERE FromKey = 1 

    UNION ALL 

    SELECT r.FromKey, r.ToKey, recursionLevel = recursionLevel + 1 
    FROM RelatedEntities r 
    INNER JOIN FindPath b ON r.FromKey = b.ToKey 
    WHERE b.ToKey <> 3 AND RecursionLevel < 10 
) 
SELECT * FROM FindPath 
ORDER BY recursionLevel 

请注意，这将从1返回到0，然后从0返回到1，并重复，直到用完递归级别。

Answer 1

您需要修改您的查询是这样的：

DECLARE @startKey UNIQUEIDENTIFIER, @endKey UNIQUEIDENTIFIER; 
DECLARE @maxRecursion INT = 100 
SET @startKey = '00000000-0000-0000-0000-000000000000'; 
SET @endKey = 'F7801327-C037-AA93-67D1-B7892F6093A7'; 

;With FindPath 
AS 
(
    SELECT FromKey, ToKey, 0 AS recursionLevel 
    FROM RelatedEntities 
    WHERE FromKey = @startKey 

    UNION ALL 

    SELECT r.FromKey, r.ToKey, recursionLevel = recursionLevel +1 
    FROM RelatedEntities r 
    INNER JOIN FindPath b ON r.FromKey = b.ToKey 
    WHERE b.ToKey <> @endKey AND recursionLevel < @maxRecursion 
) 
SELECT * FROM FindPath; 

的锚固构件上面CTE的：

SELECT FromKey, ToKey, 0 AS recursionLevel 
FROM RelatedEntities 
WHERE FromKey = @startKey 

将选择开始的记录，T0（从，到）记录链。

的CTE的递归构件：

SELECT r.FromKey, r.ToKey, recursionLevel = recursionLevel +1 
FROM RelatedEntities r 
INNER JOIN FindPath b ON r.FromKey = b.ToKey 
WHERE b.ToKey <> @endKey AND recursionLevel < @maxRecursion 

将与T0，T1，...作为输入和T1，T2被执行，...分别作为输出。

这一过程将继续添加记录到最终结果集，直到一个空集从递归构件返回时，即，直到与一个ToKey=@endKey记录已被添加到该结果集，或@maxRecursion水平已经达到。

编辑：

您可以使用下面的查询，以便在有效地处理任何循环路径：

;With FindPath 
AS 
(
    SELECT FromKey, ToKey, 
      0 AS recursionLevel, 
      CAST(FromKey AS VARCHAR(MAX)) AS FromKeys 
    FROM RelatedEntities 
    WHERE FromKey = 1 

    UNION ALL 

    SELECT r.FromKey, r.ToKey, 
      recursionLevel = recursionLevel + 1, 
      FromKeys = FromKeys + ',' + CAST(r.FromKey AS VARCHAR(MAX)) 
    FROM RelatedEntities r 
    INNER JOIN FindPath b ON r.FromKey = b.ToKey 
    WHERE (b.ToKey <> 3) 
      AND (RecursionLevel < 10) 
      AND PATINDEX('%,' + CAST(r.ToKey AS VARCHAR(MAX)) + ',%', ',' + FromKeys + ',') = 0 
) 
SELECT * FROM FindPath 
ORDER BY recursionLevel 

计算领域FromKeys用来进行FromKey下一个递归级别。通过这种方式，使用字符串连接从以前的递归级别中的任何键被级别累加。然后使用PATINDEX来检查圆形路径是否被满足。

SQL Fiddle Demo here