How can I select rows from a hierarchical query with the lowest level?
If you have an hierarchical query that produces the whole tree under the root node, that also has a level column computed, you can wrap it in a derived table or cte and use the window aggregate:

WITH query AS
  ( SELECT &lt;columns list&gt;, level
  -- your query here
  ) ,
cte AS 
  ( SELECT &lt;columns list&gt;, level,
           MIN(the_level) OVER () AS min_level
    FROM query
    WHERE &lt;conditions&gt;
  )
SELECT *
FROM cte
WHERE min_level = level ;