递归CTE找到所有项目的所有祖先

Question

我有一个简单的层次结构,需要能够生成一个表,该表将表中的EACH项与其所有祖先相匹配.(上限强调这不是一个重复的问题!)

所以这是一张桌子:

Select Item='A', Parent=null into Items union
Select Item='B', Parent='A'  union
Select Item='C', Parent='A'  union
Select Item='D', Parent='B'  union
Select Item='E', Parent='B'  union
Select Item='F', Parent='C'  union
Select Item='G', Parent='C'  union
Select Item='H', Parent='D'  
Go

...代表这种层次结构:

       A
     /   \
    B     C
   / \   / \
   D E   F G
  /
  H

所以B有一个祖先(A),H有3个祖先(D,B,A).这是所需的输出:

 Item | Ancestor
 B    | A
 C    | A
 D    | A
 D    | B
 E    | A
 E    | B
 F    | A
 F    | C
 G    | A
 G    | C
 H    | A
 H    | B
 H    | D

使用递归CTE,我能够找到任何一个项目的所有后代......

Create Function ItemDescendants(@Item char) Returns @result Table(Item char) As Begin
    ; With AllDescendants as (
        Select
            Item,
            Parent
        From Items i
        Where Item=@Item
        UNION ALL
        Select
            i.Item,
            i.Parent
        from Items i
        Join AllDescendants a on i.Parent=a.Item
    )
    Insert into @result (Item)
    Select Item from AllDescendants
    Where Item<>@Item;
    Return;
End
Go

...但是为了获得完整的扩展列表,我必须求助于光标(yuk!):

Select Item, Parent into #t From Items

Declare @Item char
Declare c Cursor for (Select Item from Items)
Open c
Fetch c into @Item
While (@@Fetch_Status=0) Begin
    Insert into #t (Item, Ancestor) Select Item, @Item from dbo.ItemDescendants(@Item) 
    Fetch c into @Item
End
Close c
Deallocate c

Select Distinct
    Item,
    Ancestor
From #t
Where Parent is not null
Order by Item,Parent

Drop Table #t

这样可行,但如果我能用一个优雅的查询来完成它,我会非常高兴.似乎它应该是可能的 - 任何想法？

Answer 1

假设我理解正确,它应该像从叶节点向后递归一样简单(这很简单,因为表项只存储叶节点):

;with AncestryTree as (
  select Item, Parent
  from Items
  where Parent is not null
  union all
  select Items.Item, t.Parent  
  from AncestryTree t 
  join Items on t.Item = Items.Parent
 )
select * from AncestryTree
order by Item, Parent

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