显示子级最高层次父级答案

【问题标题】：Display Upper Most Hierarchy Parent for Child显示子级最高层次父级
【发布时间】：2020-01-21 23:09:26
【问题描述】：

我有一个包含父子关系的表，我想为每个孩子显示最上面的父级。真实表的顺序不会如此连续地排序，因此任何类型的排序都是不可取的。我还想避免为最大数量的层次结构级别进行手动连接。

表：

Parent  Child
1       2
2       3
3       4
5       6
6       7

所以对于孩子 4 有父母 3，谁有父母 2，谁有父母 1。 4、3、2 在输出中都应该有 Parent 4。

期望的输出：

Parent       Child
1            2
1            3
1            4
5            6
5            7

【问题讨论】：

标签： sql sql-server tsql recursion common-table-expression

【解决方案1】：

;with SelectedToTopCTE as
  ( select ParentID, ChildID as Child, 1 as level
    from Table
  union all
    select d.ParentID, s.ChildID, d.level + 1
    from SelectedToTopCTE as d
      join Table s
        on d.Child = s.ParentID
  ) 
select * INTO #SelectedToTop
from SelectedToTopCTE;

SELECT Child, MAX(level) as MaxLevel INTO #UpperMostSPLT
FROM #SelectedToTop
group by Child;

SELECT A.* 
FROM #SelectedToTop A
INNER JOIN
#UpperMostSPLT B
ON A.Child = B.Child AND A.level = B.MaxLevel
ORDER BY ParentID;

【讨论】：

【解决方案2】：

递归公用表表达式 (CTE) 可以快速遍历父/子层次结构。此示例从顶级父级开始，即上面没有父级的行。然后，它一次添加一个子级，同时跟踪最顶层的父级。

-- Sample data.
declare @Samples as Table ( Parent Int, Child Int );
insert into @Samples ( Parent, Child ) values
  ( 1, 2 ), ( 2, 3 ), ( 3, 4 ), ( 5, 6 ), ( 6, 7 );
select * from @Samples;

-- Run the tree.
with Tree as (
  -- Start with the top level parents.
  select Parent as TopLevelParent, Child
    from @Samples as S
    -- Where the row has no parent above it.
    where not exists ( select 42 from @Samples as SS where S.Parent = SS.Child )
  union all
  -- Add the children one level at a time.
  select T.TopLevelParent, S.Child
    from Tree as T inner join
      @Samples as S on S.Parent = T.Child )
  -- Display the sorted results.
  select TopLevelParent, Child
    from Tree
    order by TopLevelParent, Child;

【讨论】：