SELECT a.*, b.*
FROM
    (SELECT O1.content_tree_lft, COUNT(O2.content_tree_id) AS indentation, O1.content_tree_id
   FROM content_tree AS O1, content_tree AS O2
   WHERE O1.content_tree_lft BETWEEN O2.content_tree_lft AND O2.content_tree_rgt
   GROUP BY O1.content_tree_id
   ORDER BY O1.content_tree_lft) AS a,
    (SELECT O1.content_tree_lft, COUNT(O2.content_tree_id) AS indentation, O1.content_tree_id
   FROM content_tree AS O1, content_tree AS O2
   WHERE O1.content_tree_lft BETWEEN O2.content_tree_lft AND O2.content_tree_rgt
   GROUP BY O1.content_tree_id
   ORDER BY O1.content_tree_lft) AS b
WHERE
a.indentation = b.indentation + 1
AND b.content_tree_lft = 0