Question

我有一个表（ms sql 2008），我存储用户替换。它的形式为：

| ID| U1| U2|

| 1 | A | B |

| 2 | B | C |

| 3 | C | D |

用户A被用户B替换。然后B被C替换，然后C被替换为D.替换可能是1-n。

如何将（逻辑）结果放在一行中，现在A被D？

取代

| A | D |

Answer 1

使用recursive CTE，您可以找到每行的最终替换。

WITH RCTE AS
(
    SELECT ID, U1, U2, 1 AS Lvl 
    FROM dbo.table1 --WHERE ID = 1
    UNION ALL
    SELECT r.ID, r.U1, t1.U2, r.Lvl + 1 
    FROM dbo.table1 t1
    INNER JOIN RCTE r ON r.U2 = t1.U1 AND t1.ID > r.ID
)
, CTE_RN AS 
(
    SELECT *, ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Lvl DESC) RN 
    FROM RCTE
)
SELECT ID, u1, U2 AS FinalReplacement
FROM CTE_RN c
WHERE RN = 1 
AND NOT EXISTS (SELECT * FROM dbo.table1 t1 WHERE c.u1 = t1.U2)

<强> SQLFiddle DEMO

Sql查询自引用表

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