如何对db2递归sql进行排序，以按父/子关系显示物料清单中的级别

WITH RECURSIVE cte (material_id, parent_id, level) AS (
  SELECT material_id, parent_id, 0
  FROM materials
  WHERE parent_id IS NULL
  UNION ALL
  SELECT m.material_id, m.parent_id, c.level + 1
  FROM materials m
  INNER JOIN cte c ON m.parent_id = c.material_id
)
SELECT material_id, parent_id, level
FROM cte

WITH RECURSIVE cte (material_id, parent_id, level) AS (
  SELECT material_id, parent_id, 0
  FROM materials
  WHERE parent_id IS NULL
  UNION ALL
  SELECT m.material_id, m.parent_id, c.level + 1
  FROM materials m
  INNER JOIN cte c ON m.parent_id = c.material_id
)
SELECT material_id, parent_id, level
FROM cte
ORDER BY level, parent_id