How to Find Circular References in CTE between Parent/Child tables - sql-server

Question

I have a CTE to show dependency tree over 2 tables (a parent and a child table).
There is a data problem resulting in a circular dependency, resulting in a Max Recursion level error being thrown.
i.e.
Table: Parent
Id
ItemId
Table: Child
Id
ParentId
ItemId
Example Circular Ref data
Table: Parent
Id ItemId
1 A
2 B
Table: Child
Id ParentId ItemId
1 1 B
2 2 A
There are thousands of rows in these tables. How can I write a query identify the offending reference? Or is there a way to set the Max Recursion level that will then just stop the CTE once hit instead of throw an error...then I could view the results and identify the problem child.
WITH Recursive_CTE AS
(
SELECT
ItemId,
CAST(ItemDescription AS varchar(100)) AS ItemDescription,
Qty,
CAST(ParentItemId AS SmallInt) AS ParentItemId,
CAST(ItemId AS varchar(100)) AS ParentGroupItemId,
CAST(' -' AS varchar(100)) AS LVL,
CAST(ItemId AS varchar(100)) AS HierarchyItem,
CAST(SKU AS varchar(100)) AS HierarchySKU,
CAST(ItemDescription AS varchar(100)) AS HierarchyName,
0 AS RecursionLevel
FROM dbo.vw_BOM AS child
WHERE (ParentItemId = 0)
--and ItemId = #BOMHeaderItemId
UNION ALL
SELECT
child.ItemId,
CAST(parent.LVL + child.ItemDescription AS varchar(100)) AS ItemDescription,
child.Qty,
CAST(child.ParentItemId AS SmallInt) AS ParentItemId,
parent.ParentGroupItemId,
CAST(' -' + parent.LVL AS varchar(100)) AS LVL,
CAST(parent.HierarchyItem + ':' + CAST(child.ItemId AS varchar(100)) AS varchar(100)) AS HierarchyItem,
CAST(parent.HierarchySKU + ':' + CAST(child.SKU AS varchar(100)) AS varchar(100)) AS HierarchySKU,
CAST(parent.HierarchyName + '/' + CAST(child.ItemDescription AS varchar(100)) AS varchar(100)) AS HierarchyName,
parent.RecursionLevel + 1 AS RecursionLevel
FROM Recursive_CTE AS parent INNER JOIN
dbo.vw_BOM AS child ON child.ParentItemId = parent.ItemId
)
SELECT
Recursive_CTE_1.RecursionLevel,
Recursive_CTE_1.ParentGroupItemId,
Recursive_CTE_1.ParentItemId,
Recursive_CTE_1.ItemId,
Recursive_CTE_1.Qty,
DATALENGTH(Recursive_CTE_1.LVL) AS LVLLength,
Recursive_CTE_1.ItemDescription,
item.SKU,
item.OnHandQty,
item.AllocQty,
item.AvailableQty,
item.ToBeReceivedQty,
item.AvailableWFutureQty,
Recursive_CTE_1.HierarchyItem,
Recursive_CTE_1.HierarchySKU,
Recursive_CTE_1.HierarchyName
FROM Recursive_CTE AS Recursive_CTE_1 INNER JOIN
dbo.vw_ItemInventorySummary AS item ON Recursive_CTE_1.ItemId = item.Id
ORDER BY Recursive_CTE_1.HierarchySKU
option (maxrecursion 200)
The View vw_BOM
SELECT dbo.BillOfMaterialHeader.Id AS Id, dbo.BillOfMaterialHeader.ItemId AS ItemId, 0 AS ParentItemId, FGItems.SKU AS SKU, FGItems.SKU + N': ' + FGItems.ShortDescription AS ItemDescription,
dbo.BillOfMaterialHeader.Quantity AS Qty
FROM dbo.BillOfMaterialHeader INNER JOIN
dbo.Items AS FGItems ON dbo.BillOfMaterialHeader.ItemId = FGItems.Id
UNION ALL
SELECT dbo.BillOfMaterialDetail.Id AS Id, dbo.BillOfMaterialDetail.ItemId AS ItemId, BOMHdr.ItemId AS ParentItemId, RMItems.SKU AS SKU, RMItems.SKU + N': ' + RMItems.ShortDescription AS ItemDescription,
dbo.BillOfMaterialDetail.Quantity AS Qty
FROM dbo.Items AS RMItems INNER JOIN
dbo.BillOfMaterialDetail ON RMItems.Id = dbo.BillOfMaterialDetail.ItemId INNER JOIN
dbo.BillOfMaterialHeader BOMHdr ON dbo.BillOfMaterialDetail.BillOfMaterialHeaderId = BOMHdr.Id
UPDATE
Tab's answer pointed me in the right direction. I used the flattened Parent Child table in vw_BOM and then joined it to itself per Tab's answer, which showed me where 6 Items had the same Item Id in the Parent Table and the Child Table.
Like so:
SELECT dbo.vw_BOM.SKU AS ParentSKU, vw_BOM_1.SKU AS ChildSKU
FROM dbo.vw_BOM INNER JOIN
dbo.vw_BOM AS vw_BOM_1 ON dbo.vw_BOM.ItemId = vw_BOM_1.ParentItemId AND dbo.vw_BOM.ParentItemId = vw_BOM_1.ItemId

Answer 1

Your CTE already has a hierarchy with the ItemID path concatenated. How about using that to determine if the item has already been seen?
Add a new column to the anchor portion of your CTE, HasCycle = Convert(bit, 0).
Then in the recursive portion of your CTE, add the column and a condition in the WHERE clause like so:
...
UNION ALL
SELECT
... other columns,
HasCycle = Convert(bit,
CASE
WHEN ':' + parent.HierarchyItem + ':' LIKE
'%:' + Convert(varchar(100), child.ItemID) + ':%'
THEN 1
ELSE 0
END)
FROM
...
WHERE
...
AND parent.HasCycle = 0 --terminate after cycle is found
;
Then you can select from the recursive CTE WHERE HasCycle = 1 and see all the rows that begin a cycle and their exact path upwards in the HierarchyItem.

Answer 2

Simple self-join should do it:
SELECT * FROM MyTable t1
INNER JOIN MyTable t2
ON t1.Parent=t2.Child
AND t1.Child=t2.Parent

Answer 3

I have seen these issues before and have resorted to adding items one level at a time, ignoring those seen before.