CTE đệ quy: phân cấp độ sâu tùy ý

WITH RECURSIVE nome (col1, col2, …) AS (
  -- (1) Termine BASE: le righe di partenza
  SELECT …
  FROM   …
  WHERE  …

  UNION ALL

  -- (2) Termine RICORSIVO: si appoggia a "nome" stesso per espandere
  SELECT …
  FROM   tabella  t
  JOIN   nome    n  ON …
)
SELECT * FROM nome;

WITH RECURSIVE tree AS (
  -- base: le radici (parent_id IS NULL)
  SELECT id,
         name,
         parent_id,
         1                 AS depth,
         name              AS path
  FROM   categories
  WHERE  parent_id IS NULL

  UNION ALL

  -- ricorsivo: per ogni categoria che ha come parent una riga già in tree
  SELECT c.id,
         c.name,
         c.parent_id,
         t.depth + 1,
         t.path || ' > ' || c.name
  FROM   categories c
  JOIN   tree       t ON c.parent_id = t.id
)
SELECT id, name, depth, path
FROM   tree
ORDER BY path;

WITH RECURSIVE tree AS (
  SELECT id,
         name,
         parent_id,
         id   AS root_id,
         name AS root_name
  FROM   categories
  WHERE  parent_id IS NULL

  UNION ALL

  SELECT c.id,
         c.name,
         c.parent_id,
         t.root_id,    -- propago lo stesso id_radice
         t.root_name   -- e nome
  FROM   categories c
  JOIN   tree       t ON c.parent_id = t.id
)
SELECT id, name, root_name
FROM   tree
ORDER BY id;