Learing SQL, and have a bit of a problem. I have 2 tables level and level_hierarchy
|name | id | |parent_id | child_id|
------------------- ---------------------
| Level1_a | 1 | | NULL | 1 |
| Level2_a | 19 | | 1 | 19 |
| Level2_b | 3 | | 1 | 3 |
| Level3_a | 4 | | 3 | 4 |
| Level3_b | 5 | | 3 | 5 |
| Level4_a | 6 | | 5 | 6 |
| Level4_b | 7 | | 5 | 7 |
Now what I need, is a query that will return all entries from table level from every hirarchy level based on parameter that marks what level hierarchy level I want to get entries from.
Getting Level1 entries is quite easy.
SELECT name FROM level INNER JOIN level_hierarchy ON level.id =
level_hierarchy.child_id WHERE level_hierarchy.parent_id=NULL
Level2 entries:
Level2_a
Level2_b
are just the ones that have a parent and the parent of their parent is NULL and so on. This is where I suspect that recursion comes in.
Is there anyone who can guide thorugh it?
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)
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 *
