Recursive query for hirarchical data based on adjacency list

Question

Learing SQL, and have a bit of a problem. I have 2 tables level and level_hierarchy

|name        | id |     |parent_id | child_id|
         


        

Answer 1

Your query for the first level (here depth to distinguish from the table) should look like this:

select l.name, h.child_id, 1 as depth 
from level l
join level_hierarchy h on l.id = h.child_id 
where h.parent_id is null;

   name   | child_id | depth 
----------+----------+-------
 Level1_a |        1 |     1
(1 row)

Note the proper use of is null (do not use = to compare with null as it always gives null).

You can use the above as an initial query in a recursive cte:

with recursive recursive_query as (
    select l.name, h.child_id, 1 as depth 
    from level l
    join level_hierarchy h on l.id = h.child_id 
    where h.parent_id is null
union all
    select l.name, h.child_id, depth + 1
    from level l
    join level_hierarchy h on l.id = h.child_id
    join recursive_query r on h.parent_id = r.child_id
)
select *
from recursive_query
-- where depth = 2

   name   | child_id | depth 
----------+----------+-------
 Level1_a |        1 |     1
 Level2_b |        3 |     2
 Level2_a |       19 |     2
 Level3_a |        4 |     3
 Level3_b |        5 |     3
 Level4_a |        6 |     4
 Level4_b |        7 |     4
(7 rows)    

Answer 2

Good question, recursion is a difficult topic in SQL and its implementation varies by engine. Thanks for tagging your post with PostgreSQL. PostgreSQL has some excellent documentation on the topic.

WITH RECURSIVE rec_lh(child_id, parent_id) AS (
    SELECT child_id, parent_id FROM level_hierarchy
  UNION ALL
    SELECT lh.child_id, lh.parent_id
    FROM rec_lh rlh INNER JOIN level_hierarchy lh
      ON lh.parent_id = rlh.child_id
  )
SELECT DISTINCT level.name, child_id 
FROM rec_lh INNER JOIN level
  ON rec_lh.parent_id = level.id
ORDER BY level.name ASC;

See Also:

Recursive query in PostgreSQL. SELECT *