sql - 在 SQL 中查找顶级父级

Question

我有两个表如下

表人

  Id   Name
   1    A
   2    B
   3    C
   4    D
   5    E

表RelationHierarchy

ParentId   ChildId
   2         1
   3         2
   4         3

这将形成一个树状结构

          D
          |
          C
          |
          B
          |
          A

ParentId和ChildId是Person表的Id列的外键

我需要编写可以获取顶级父级(即根)的 SQL。任何人都可以建议任何可以帮助我完成此任务的 SQL

Answer 1

您可以使用recursive CTE实现这一目标:

DECLARE @childID INT 
SET @childID  = 1 --chield to search

;WITH RCTE AS
(
    SELECT *, 1 AS Lvl FROM RelationHierarchy 
    WHERE ChildID = @childID

    UNION ALL

    SELECT rh.*, Lvl+1 AS Lvl FROM dbo.RelationHierarchy rh
    INNER JOIN RCTE rc ON rh.CHildId = rc.ParentId
)
SELECT TOP 1 id, Name
FROM RCTE r
inner JOIN dbo.Person p ON p.id = r.ParentId
ORDER BY lvl DESC

<强> SQLFiddle DEMO

编辑 - 针对所有 child 的顶级家长的更新请求:

;WITH RCTE AS
(
    SELECT  ParentId, ChildId, 1 AS Lvl FROM RelationHierarchy 

    UNION ALL

    SELECT rh.ParentId, rc.ChildId, Lvl+1 AS Lvl 
    FROM dbo.RelationHierarchy rh
    INNER JOIN RCTE rc ON rh.ChildId = rc.ParentId
)
,CTE_RN AS 
(
    SELECT *, ROW_NUMBER() OVER (PARTITION BY r.ChildID ORDER BY r.Lvl DESC) RN
    FROM RCTE r

)
SELECT r.ChildId, pc.Name AS ChildName, r.ParentId, pp.Name AS ParentName
FROM CTE_RN r
INNER JOIN dbo.Person pp ON pp.id = r.ParentId
INNER JOIN dbo.Person pc ON pc.id = r.ChildId
WHERE RN =1

<强> SQLFiddle DEMO

EDIT2 - 让所有人在最后稍微改变一下 JOINS:

SELECT pc.Id AS ChildID, pc.Name AS ChildName, r.ParentId, pp.Name AS ParentName
FROM dbo.Person pc 
LEFT JOIN CTE_RN r ON pc.id = r.CHildId AND  RN =1
LEFT JOIN dbo.Person pp ON pp.id = r.ParentId

SQLFiddle DEMo