Trigger to prevent infinite loop in a sql tree - sql

Question

I have node table with (NodeId, NodeName) and structure table (ParentNodeId, ChildNodeId). How can I write a trigger that check if an insert update or delete statement can cause infinite relation?

Answer 1

Here is my solution, and so far it works as expected.
CREATE TRIGGER [dbo].[CheckNodeDependence] ON [dbo].[ObjectTrees]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
DECLARE #CTable TABLE(ChildId INT NOT NULL,
ParentId INT NOT NULL,
[Level] INT NOT NULL,
RowId INT NOT NULL)
DECLARE #Level INT
SET #Level = 1
DECLARE #rows_affected INT
SET #rows_affected = 1
INSERT INTO #CTable
SELECT ObjectId, ParentId, 1, ObjectId FROM INSERTED
WHILE #rows_affected > 0
BEGIN
SET #Level = #Level + 1
INSERT INTO #CTable
SELECT T.ObjectId, T.ParentId, #Level, C.RowId
FROM ObjectTrees T
INNER JOIN #CTable C ON T.ParentId = C.ChildId
AND C.Level = #Level - 1
SET #rows_affected = ##rowcount
IF EXISTS(
SELECT * FROM #CTable B
INNER JOIN #CTable V ON B.level = 1
AND V.Level > 1
AND V.RowId = B.RowId
AND V.ChildId = B.RowId)
BEGIN
DECLARE #error_message VARCHAR(200)
SET #error_message = 'Operation would cause illegal circular reference in tree structure, level = ' + CAST(#Level AS VARCHAR(30))
RAISERROR(#error_message,16,1)
ROLLBACK TRANSACTION
RETURN
END
END
END
GO

Answer 2

You'll have to recursively check the circular dependency condition in which the parent doesn't become a child of its own child, either directly or indirectly.
In SQL Server 2005, you can write a recursive CTE for the same. An example -
WITH [RecursiveCTE]([Id], [ParentAccountId]) AS
(
SELECT
[Id],
[ParentAccountId]
FROM [Structure]
WHERE [Id] = #Id
UNION ALL
SELECT
S.[Id],
S.[ParentAccountId]
FROM [Structure] S INNER JOIN [RecursiveCTE] RCTE ON S.[ParentAccountId] = RCTE.[Id]
)
SELECT * FROM [RecursiveCTE]