SQL Server 2017

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CREATE TABLE dbo.Levels ([PARTID] INT,[ROOT] int,[SUB] int,[LEVEL] int, [LEADTIME] int,[TOP LEVEL PART] int);
--PartID added
INSERT INTO dbo.Levels
 ([PARTID], [ROOT] ,[SUB] ,[LEVEL] , [LEADTIME] ,[TOP LEVEL PART])

VALUES
(1,1 , 1 ,0 ,4 ,1 ),
(1,1 , 2 ,1 ,2 ,1 ),
(1,1 , 3 ,1 ,5 ,1 ),
(1,1 , 7 ,1 ,20 ,1 ),
(1,2 , 4 ,1 ,4 ,1 ),
(1,2 , 5 ,1 ,1 ,1 ),
(1,3 , 4 ,2 ,4 ,1 ),
(1,3 , 6 ,2 ,5 ,1 );
8 rows affected

;WITH RecursiveCTE
AS
(
SELECT	[SUB] , 
		[SUB] as [Top Level Part],
		CAST([SUB] as varchar(255)) as [Parts Nodes Combination],
		0 AS [LEVEL],
		[LEADTIME]  as [Days],
		CAST([LEADTIME] as varchar(255)) as [BASED ON]
FROM dbo.Levels s1
WHERE [LEVEL] = 0

UNION ALL

SELECT	s1.SUB as [Top Level Part],
		RecursiveCTE.[Top Level Part],
		CAST(RecursiveCTE.[Parts Nodes Combination]+ ' >> ' + cast(s1.[SUB] as varchar(255)) as varchar(255)),
		RecursiveCTE.[LEVEL] + 1 as [LEVEL],
		RecursiveCTE.[Days] + s1.[LEADTIME] as [Days],
		CAST(RecursiveCTE.[BASED ON] + ' + ' + CAST(s1.[LEADTIME] as varchar(255))  as  varchar(255))
FROM dbo.Levels s1
INNER JOIN RecursiveCTE 
ON s1.[ROOT] = RecursiveCTE.[SUB]
WHERE s1.[LEVEL] > 0
)
SELECT * FROM RecursiveCTE
WHERE [LEVEL] != 0;


SUB
Top Level Part
Parts Nodes Combination
LEVEL
Days
BASED ON


2
1
1 >> 2
1
6
4 + 2

3
1
1 >> 3
1
9
4 + 5

7
1
1 >> 7
1
24
4 + 20

4
1
1 >> 3 >> 4
2
13
4 + 5 + 4

6
1
1 >> 3 >> 6
2
14
4 + 5 + 5

4
1
1 >> 2 >> 4
2
10
4 + 2 + 4

5
1
1 >> 2 >> 5
2
7
4 + 2 + 1

SUB	Top Level Part	Parts Nodes Combination	LEVEL	Days	BASED ON
2	1	1 >> 2	1	6	4 + 2
3	1	1 >> 3	1	9	4 + 5
7	1	1 >> 7	1	24	4 + 20
4	1	1 >> 3 >> 4	2	13	4 + 5 + 4
6	1	1 >> 3 >> 6	2	14	4 + 5 + 5
4	1	1 >> 2 >> 4	2	10	4 + 2 + 4
5	1	1 >> 2 >> 5	2	7	4 + 2 + 1