SQL Update Parent Child relation on Trigger - sql-server

Question

I would like to link rows in a self joined table with a trigger.
After an insert in a root table, I would like to create 3 "levels" in the child table.
And each of the level being a hierarchical data (or self joined) such as:
LVL1
LVL2
LVL3
Database is SQLSERVER.
I know there is a ton of material about self-joined and hierarchical SQL data, but ... I don't know I've not found what I expected. I've spent too many hours trying solutions, and searching online.
A link to SQLFiddle.
Here is the basic schema for example:
CREATE TABLE [dbo].[Root] (
[RootID] [int] PRIMARY KEY IDENTITY(1,1) NOT NULL,
[Name] [varchar](50)
)
GO
CREATE TABLE [dbo].[Child] (
[ChildID] [int] PRIMARY KEY IDENTITY(1,1) NOT NULL,
[RootID] [int],
[Name] [varchar](50),
[ParentID] [int]
)
GO
ALTER TABLE [dbo].[Child] WITH CHECK ADD CONSTRAINT [Child_RootID_FK] FOREIGN KEY([RootID])
REFERENCES [dbo].[Root] ([RootID]) ON DELETE SET NULL
GO
ALTER TABLE [dbo].[Child] WITH CHECK ADD CONSTRAINT [Child_ParentID_FK] FOREIGN KEY([ParentID])
REFERENCES [dbo].[Child] ([ChildID])
GO
CREATE TRIGGER [dbo].[Root_TR]
ON [dbo].[Root]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
INSERT INTO [dbo].[Child] ([RootID], [Name])
SELECT
I.[RootID],
CONCAT_WS('_', (SELECT [Name] FROM [dbo].[Root] R WHERE R.[RootID] = I.RootID), LVL.n )
FROM INSERTED I
CROSS JOIN (VALUES (1), (2), (3)) AS LVL(n)
END
GO
INSERT INTO [dbo].[Root] ([Name]) VALUES (
'Foo'
)
SELECT * FROM [Root]
SELECT * FROM [Child]
I have the current result:
ChildID
RootID
Name
ParentID
1
1
Foo_1
NULL
2
1
Foo_2
NULL
3
1
Foo_3
NULL
The expected result would be:
ChildID
RootID
Name
ParentID
1
1
Foo_1
NULL
2
1
Foo_2
1
3
1
Foo_3
2
I'm not sure how to achieve this.
I've found a close answer (here) involving usage of SEQUENCE
A solution may be something like:
DROP TRIGGER [dbo].[Root_TR]
GO
CREATE SEQUENCE [dbo].[Sequence] START WITH 1 INCREMENT BY 1
GO
CREATE TRIGGER [dbo].[Root_TR]
ON [dbo].[Root]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
ALTER SEQUENCE [dbo].[Sequence] RESTART
DECLARE #map TABLE ([ID] [int], [Seq] [int])
INSERT INTO [dbo].[Child] ([RootID], [Name])
OUTPUT [inserted].ChildID, NEXT VALUE FOR [dbo].[Sequence] INTO #map
SELECT
I.[RootID],
CONCAT_WS('_', (SELECT [Name] FROM [dbo].[Root] R WHERE R.[RootID] = I.RootID), LVL.n )
FROM INSERTED I
CROSS JOIN (VALUES (1), (2), (3)) AS LVL(n)
UPDATE C
SET C.[ParentID] = CASE
WHEN M.Seq = 1
THEN NULL
ELSE
(SELECT [Id] FROM #map WHERE [Seq] = [Seq] - 1)
END
FROM [dbo].Child C
INNER JOIN #map M ON C.ChildID = M.ID
END
GO
Unfortunately usage of NEXT VALUE FOR is not allowed in a OUTPUT clause.
Error 11720 NEXT VALUE FOR function is not allowed in the TOP, OVER, OUTPUT, ON, WHERE, GROUP BY, HAVING, or ORDER BY clauses.
I cannot relie on [Name] column to perform the UPDATE SET [ParentID] = ... FROM ... JOIN ...
There is a lot of answers regarding SQL self joined table but I can't really find the answer and my knowledge regarding SQL is limited.
I've attempted something like this
CREATE TRIGGER [dbo].[Root_TR]
ON [dbo].[Root]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
ALTER SEQUENCE [dbo].[Sequence] RESTART
DECLARE #map TABLE ([ChildID] [int], [Seq] [int])
DECLARE #i int = NEXT VALUE FOR [Sequence]
INSERT INTO [dbo].[Child] ([RootID], [Name])
OUTPUT [inserted].ChildID, #i AS [Seq] INTO #map
SELECT
I.[RootID],
CONCAT_WS('_', (SELECT [Name] FROM [dbo].[Root] R WHERE R.[RootID] = I.RootID), LVL.n )
FROM INSERTED I
CROSS JOIN (VALUES (1), (2), (3)) AS LVL(n)
DECLARE #xml xml = (SELECT * FROM #map FOR XML AUTO)
PRINT CONVERT(nvarchar(max), #xml)
UPDATE C
SET C.[ParentID] = CASE
WHEN M.Seq = 1
THEN NULL
ELSE
(SELECT [ChildID] FROM #map WHERE [Seq] = [Seq] - 1)
END
FROM [dbo].Child C
INNER JOIN #map M ON C.ChildID = M.ChildID
END
GO
Once again unfortunately the temporary TABLE #map is not filled correctly.
The #i is called once and do not increment for each output row.
ChildID
Seq
10
1
11
1
12
1
Another try was to use a DEFAULT. But I've an error when creating the trigger: "Column name or number of supplied values does not match table definition.". Probably because the OUTPUT clause see more columns available in #map TABLE than the number of columns in the OUTPUT.
CREATE TRIGGER [dbo].[Root_TR]
ON [dbo].[Root]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
ALTER SEQUENCE [dbo].[Sequence] RESTART
DECLARE #map TABLE (
[Seq] [int] PRIMARY KEY NOT NULL DEFAULT (NEXT VALUE FOR [Sequence]),
[ChildID] [int]
)
INSERT INTO [dbo].[Child] ([RootID], [Name])
OUTPUT [inserted].ChildID INTO #map
SELECT
I.[RootID],
CONCAT_WS('_', (SELECT [Name] FROM [dbo].[Root] R WHERE R.[RootID] = I.RootID), LVL.n )
FROM INSERTED I
CROSS JOIN (VALUES (1), (2), (3)) AS LVL(n)
DECLARE #xml xml = (SELECT * FROM #map FOR XML AUTO)
PRINT CONVERT(nvarchar(max), #xml)
UPDATE C
SET C.[ParentID] = CASE
WHEN M.Seq = 1
THEN NULL
ELSE
(SELECT [ChildID] FROM #map WHERE [Seq] = [Seq] - 1)
END
FROM [dbo].Child C
INNER JOIN #map M ON C.ChildID = M.ChildID
END
GO
And the last try was to use a #table (I do not distinguish clearly between #x TABLE, TABLE #x, TABLE x). But I want to scope the temporary table to the trigger, and not make the table available globally.
So, this time the TRIGGER is created. But when fired, the error message is Invalid object name 'Sequence'. I don't know why the [Sequence] object is not available in this #table.
CREATE TRIGGER [dbo].[Root_TR]
ON [dbo].[Root]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
ALTER SEQUENCE [dbo].[Sequence] RESTART
CREATE TABLE #map (
[Seq] [int] PRIMARY KEY NOT NULL DEFAULT (NEXT VALUE FOR [Sequence]),
[ChildID] [int] NOT NULL
)
INSERT INTO [dbo].[Child] ([RootID], [Name])
OUTPUT [inserted].ChildID INTO #map
SELECT
I.[RootID],
CONCAT_WS('_', (SELECT [Name] FROM [dbo].[Root] R WHERE R.[RootID] = I.RootID), LVL.n )
FROM INSERTED I
CROSS JOIN (VALUES (1), (2), (3)) AS LVL(n)
DECLARE #xml xml = (SELECT * FROM #map FOR XML AUTO)
PRINT CONVERT(nvarchar(max), #xml)
UPDATE C
SET C.[ParentID] = CASE
WHEN M.Seq = 1
THEN NULL
ELSE
(SELECT [ChildID] FROM #map WHERE [Seq] = [Seq] - 1)
END
FROM [dbo].Child C
INNER JOIN #map M ON C.ChildID = M.ChildID
END
GO
So finally I don't have yet any solution to properly link my different levels with their parents.
Any help would be highly appreciated.

Answer 1

Here the complete trigger.
CREATE TRIGGER [dbo].[Root_TR]
ON [dbo].[Root]
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON
CREATE TABLE #map (
[Seq] [int] PRIMARY KEY NOT NULL IDENTITY(1,1)
[ChildID] [int] NOT NULL
)
INSERT INTO [dbo].[Child] ([RootID], [Name])
OUTPUT INSERTED.ChildID INTO #map
SELECT
I.[RootID],
CONCAT_WS('_', (SELECT [Name] FROM [dbo].[Root] R WHERE R.[RootID] = I.RootID), LVL.n )
FROM INSERTED I
CROSS JOIN (VALUES (1), (2), (3)) AS LVL(n)
UPDATE C
SET C.[ParentID] = CASE
WHEN M.[Seq] = 1
THEN NULL
ELSE
(SELECT [ChildID] FROM #map WHERE [Seq] = M.[Seq] - 1)
END
FROM [dbo].Child C
INNER JOIN #map M ON C.ChildID = M.ChildID
END
GO