CREATE TABLE #temp
(
	article varchar(20) NOT NULL
	,part varchar(20) NOT NULL
);
INSERT INTO #temp
VALUES ('article1', 'article1a')
	,('article1', 'article1b')
	,('article2', 'article1a')
	,('article2', 'article2b')
	,('article2b', 'article3a')
	,('article2b', 'article3b');

WITH Parts
AS
(
	SELECT DISTINCT
		CAST(NULL AS varchar(20)) AS parent
		,article
		,1 AS hlevel
	FROM #temp A
	WHERE NOT EXISTS
	(
		SELECT 1
		FROM #temp A1
		WHERE A1.part = A.article
	)

	UNION ALL

	SELECT A.article, A.part, P.hlevel + 1
	FROM #temp A
		JOIN Parts P
			ON A.article = P.article
)
,PartNos
AS
(
	SELECT
		ROW_NUMBER() OVER (ORDER BY hlevel, parent, article) AS RN
		,parent, article
	FROM Parts
	WHERE parent IS NOT NULL
)
SELECT P.RN, P1.RN AS ParentRN
	,P.parent as article
	,P.article as part
FROM PartNos P
	LEFT JOIN PartNos P1
		ON P.parent = P1.article
ORDER BY RN;