DECLARE @TopLevelId	INT;
SET @TopLevelId = 23;

WITH CTE AS
(SELECT id, name, parentid, id as Adam, 1 as level
FROM Category
WHERE id = @TopLevelId
	UNION ALL
SELECT Category.id, Category.name, Category.parentid, CTE.Adam, CTE.level + 1
FROM Category 
	INNER JOIN CTE ON
		Category.parentid = CTE.id
WHERE Category.id <> @TopLevelId
),
Prods AS
(SELECT productid, 
	description, 
	categoryId,
	NewId() as GUID_
FROM Products
),
Prods5 AS
(SELECT  productid, 
	description, 
	categoryId,
	ROW_NUMBER() OVER (PARTITION by categoryId ORDER BY GUID_) as RowNum
FROM Prods
)
SELECT CTE.id, CTE.name, Prods5.productid, Prods5.description
FROM CTE
	LEFT OUTER JOIN Prods5 ON
		CTE.id = Prods5.categoryId AND
		Prods5.RowNum <= 5
ORDER BY level, CTE.id