Sql 原始零级（0）的值：

给出了表1的以下邻接列表：

| parent_id | id |
|-----------|----|
|    (null) |  1 |
|    (null) |  2 |
|         1 |  3 |
|         3 |  4 |
|         1 |  5 |
|         2 |  6 |
|         6 |  7 |
|         7 |  8 |
|         9 | 10 |
|        10 | 11 |
|        11 |  9 |

上面的查询工作在SQL Server（以及Oracle、PostgreSQL和MySQL 8，当按照指示修改时）上，它正确地检测到节点9、10和11参与长度为3的循环

SQL（/DB）小提琴在各种数据库中演示了这一点，可以在下面找到：

---

如果问题远远不够完美，甚至不要试图给出完美的答案。感谢您的努力。您使用的是哪种数据库管理系统？博士后？Oracle？最干净的解决方案。同意@adrian mitev的观点：这是优雅的简单；应该是公认的答案，依我看。@VictoriaStuart:公认的答案是针对SQL Server的，尽管Interstellar从未证实过这一点，但可以安全地假设她/他正在使用它，所以这个答案可能不合适。鉴于我有多个父层次结构（），我有没有选择使用类似的解决方案？显然，我无法将

id

添加到

所有父节点

，因为这对于我的多个父节点层次结构中的不同

父节点

可能仍然有效。@NeverEndingQueue:您需要在检查中包括根节点的id。查看我的编辑（未测试）如果循环导致子对象永远不等于孙子对象，该怎么办？即，周期长于2个节点？那么这个解决方案还有效吗？@avl_瑞典是的。该解决方案利用了

级别

计数器，也适用于没有

数组

功能的SQL方言（与Postgres不同）。例如，对我来说，它与雪花一起工作+谢谢你！

-- Some random data
IF OBJECT_ID('tempdb..#MyTable') IS NOT NULL
    DROP TABLE #MyTable

CREATE TABLE #MyTable (ID INT PRIMARY KEY, ParentID INT NULL, Description VARCHAR(100))
INSERT INTO #MyTable (ID, ParentID, Description) VALUES
(1, NULL, 'Parent'), -- Try changing the second value (NULL) to 1 or 2 or 3
(2, 1, 'Child'), -- Try changing the second value (1) to 2 
(3, 3, 'SubChild')
-- End random data

-- The "terminal" childrens (that are elements that don't have childrens
-- connected to them)
;WITH WithoutChildren AS
(
    SELECT MT1.* FROM #MyTable MT1
        WHERE NOT EXISTS (SELECT 1 FROM #MyTable MT2 WHERE MT1.ID != MT2.ID AND MT1.ID = MT2.ParentID)
)

, RecursiveCTE (StartingID, Level, Parents, Descriptions, Loop, ParentID) AS
(
    SELECT ID, -- StartingID 
        1, -- Level
        '|' + CAST(ID AS VARCHAR(MAX)) + '|', 
        '|' + CAST(Description AS VARCHAR(MAX)) + '|', 
        0, -- Loop
        ParentID
        FROM WithoutChildren
    UNION ALL
    SELECT R.StartingID, -- StartingID
        R.Level + 1, -- Level
        R.Parents + CAST(MT.ID AS VARCHAR(MAX)) + '|',
        R.Descriptions + CAST(MT.Description AS VARCHAR(MAX)) + '|', 
        CASE WHEN R.Parents LIKE '%|' + CAST(MT.ID AS VARCHAR(MAX)) + '|%' THEN 1 ELSE 0 END,
        MT.ParentID
        FROM #MyTable MT
        INNER JOIN RecursiveCTE R ON R.ParentID = MT.ID AND R.Loop = 0
)

SELECT * FROM RecursiveCTE 
    WHERE ParentID IS NULL OR Loop = 1

declare @LinkTable table (Parent int, Child int);
insert into @LinkTable values (1, 2), (1, 3), (2, 4), (2, 5), (3, 6), (3, 7), (7, 1);

with cte as (
    select lt1.Parent, lt1.Child, lt2.Child as Grandchild
    from @LinkTable lt1
    inner join @LinkTable lt2 on lt2.Parent = lt1.Child
    union all
    select cte.Parent, lt1.Child, lt3.Child as Grandchild
    from cte
    inner join @LinkTable lt1 on lt1.Parent = cte.Child
    inner join @LinkTable lt2 on lt2.Parent = cte.Grandchild
    inner join @LinkTable lt3 on lt3.Parent = lt2.Child
    where cte.Child <> cte.Grandchild
)
select Parent, Child
from cte
where Child = Grandchild;

create table hierarchy (id integer, parent_id integer);

insert into hierarchy
values
(1, null), -- root element
(2, 1), -- first child
(3, 1), -- second child
(4, 3), 
(5, 4), 
(3, 5); -- endless loop

with recursive tree as (
  select id, 
         parent_id, 
         array[id] as all_parents
  from hierarchy
  where parent_id is null

  union all

  select c.id, 
         c.parent_id,
         p.all_parents||c.id
  from hierarchy c
     join tree p
      on c.parent_id = p.id 
     and c.id <> ALL (p.all_parents) -- this is the trick to exclude the endless loops
)
select *
from tree;

with recursive tree as (
  select id, 
         parent_id, 
         array[id] as all_parents, 
         id as root_id
  from hierarchy
  where parent_id is null

  union all

  select c.id, 
         c.parent_id,
         p.all_parents||c.id, 
         p.root_id
  from hierarchy c
     join tree p
      on c.parent_id = p.id 
     and c.id <> ALL (p.all_parents) -- this is the trick to exclude the endless loops
     and c.root_id = p.root_id
)
select *
from tree;

WITH EMP_CTE AS
( 

    SELECT 
        0 AS [LEVEL],   
        ManagerId, EmployeeId, Name
    FROM Employees
    WHERE ManagerId IS NULL

    UNION ALL

    SELECT 
        [LEVEL] + 1 AS [LEVEL],
        ManagerId, EmployeeId, Name
    FROM Employees e
    INNER JOIN EMP_CTE c ON e.ManagerId = c.EmployeeId 
 AND s.LEVEL < 100 --RECURSION LIMIT
) 

    SELECT  * FROM EMP_CTE WHERE [Level] = 100

CREATE TABLE MyTable
(
    [ID] INT,
    [ParentID] INT,
    [Name] NVARCHAR(255)
);

INSERT INTO MyTable
(
    [ID],
    [ParentID],
    [Name]
)
VALUES
(1, NULL, 'A root'),
(2, NULL, 'Another root'),
(3, 1, 'Child of 1'),
(4, 3, 'Grandchild of 1'),
(5, 4, 'Great grandchild of 1'),
(6, 1, 'Child of 1'),
(7, 8, 'Child of 8'),
(8, 7, 'Child of 7'), -- This will cause infinite recursion
(9, 1, 'Child of 1');

;WITH RecursiveCTE
AS (
   -- Get all parents: 
   -- Any record in MyTable table could be an Parent
   -- We don't know here yet which record can involve in an infinite recursion.
   SELECT ParentID AS StartID,
          ID,
          CAST(Name AS NVARCHAR(255)) AS [ParentChildRelationPath]
   FROM MyTable
   UNION ALL

   -- Recursively try finding all the childrens of above parents
   -- Keep on finding it until this child become parent of above parent.
   -- This will bring us back in the circle to parent record which is being
   -- keep in the StartID column in recursion
   SELECT RecursiveCTE.StartID,
          t.ID,
          CAST(RecursiveCTE.[ParentChildRelationPath] + ' -> ' + t.Name AS NVARCHAR(255)) AS [ParentChildRelationPath]
   FROM RecursiveCTE
       INNER JOIN MyTable AS t
           ON t.ParentID = RecursiveCTE.ID
   WHERE RecursiveCTE.StartID != RecursiveCTE.ID)

-- FInd the ones which causes the infinite recursion
SELECT StartID,
       [ParentChildRelationPath],
       RecursiveCTE.ID
FROM RecursiveCTE
WHERE StartID = ID
OPTION (MAXRECURSION 0);

-- For PostgreSQL and MySQL 8 use the Recursive key word in the CTE code:
-- with RECURSIVE cte(ancestor, child, lev, cycle) as (

with cte(ancestor, child, lev, cycle) as (
  select id, id, 0, 0 from Table1
  union all
  select cte.ancestor
       , Table1.id
       , case when cte.ancestor = Table1.id then 0 else cte.lev + 1 end
       , case when cte.ancestor = Table1.id then cte.lev + 1 else 0 end
    from Table1
    join cte
      on cte.child = Table1.PARENT_ID
   where cte.cycle = 0
) -- In oracle uncomment the next line
-- cycle child set isCycle to 'Y' default 'N'
select distinct
       ancestor
     , child
     , lev
     , max(cycle) over (partition by ancestor) cycle
  from cte

| parent_id | id |
|-----------|----|
|    (null) |  1 |
|    (null) |  2 |
|         1 |  3 |
|         3 |  4 |
|         1 |  5 |
|         2 |  6 |
|         6 |  7 |
|         7 |  8 |
|         9 | 10 |
|        10 | 11 |
|        11 |  9 |