POSTGRESQL Recursive LEFT JOIN on Array - postgresql

Question

I have the following table with the parent_path array:
Id | Account Name | parent_path
1 A {1}
2 B {2,1}
3 C {3,2,1}
4 D {4,3,2,1}
What I'm looking to do is to have a recursive left join in order to create 1 column per item in the parent_path array
Id | Account Name | parent_path | parent_name1 | parent_name2 | parent_name3
1 A NULL NULL NULL NULL
2 B {1} A NULL NULL
3 C {2,1} B A NULL
4 D {3,2,1} C B A
Thanks!

Answer 1

This is an abuse of SQL, but here goes:
with get_names as (
select h.id, h.account_name, h.parent_path, array_agg(h2.account_name order by p.rn) as name_path
from hier h
cross join lateral unnest(h.parent_path) with ordinality as p(path_id, rn)
join hier h2 on h2.id = p.path_id
group by h.id, h.account_name, h.parent_path
)
select id, account_name, parent_path,
name_path[2] as parent_name1,
name_path[3] as parent_name2,
name_path[4] as parent_name3,
name_path[5] as parent_name4,
name_path[6] as parent_name5,
name_path[7] as parent_name6,
name_path[8] as parent_name7,
name_path[9] as parent_name8
from get_names;
id | account_name | parent_path | parent_name1 | parent_name2 | parent_name3 | parent_name4 | parent_name5 | parent_name6 | parent_name7 | parent_name8
----+--------------+-------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------+--------------
1 | A | {1} | | | | | | | |
2 | B | {2,1} | A | | | | | | |
3 | C | {3,2,1} | B | A | | | | | |
4 | D | {4,3,2,1} | C | B | A | | | | |
(4 rows)

Answer 2

There is a cleaner solution, using PostgreSQL intarray, instead. It works best for small(ish) tables, since it's not optimised for performance:
CREATE EXTENSION intarray;
SELECT id,
name,
path - id as parents,
(SELECT name FROM hierarchy h2 WHERE h2.id = (h.path - h.id)[1]) as parent_1,
(SELECT name FROM hierarchy h2 WHERE h2.id = (h.path - h.id)[2]) as parent_2,
(SELECT name FROM hierarchy h2 WHERE h2.id = (h.path - h.id)[3]) as parent_3,
(SELECT name FROM hierarchy h2 WHERE h2.id = (h.path - h.id)[4]) as parent_4,
(SELECT name FROM hierarchy h2 WHERE h2.id = (h.path - h.id)[5]) as parent_5,
(SELECT name FROM hierarchy h2 WHERE h2.id = (h.path - h.id)[6]) as parent_6,
(SELECT name FROM hierarchy h2 WHERE h2.id = (h.path - h.id)[7]) as parent_7
FROM hierarchy h
In my code, it produced the following (truncated) output:
+--+-----------+-------+--------+--------+--------+
|id|name |parents|parent_1|parent_2|parent_3|
+--+-----------+-------+--------+--------+--------+
|1 |Europe | |NULL |NULL |NULL |
|2 |Germany |{1} |Europe |NULL |NULL |
|4 |Netherlands|{1} |Europe |NULL |NULL |
|7 |Africa | |NULL |NULL |NULL |
|10|France |{1} |Europe |NULL |NULL |
|12|America | |NULL |NULL |NULL |
|17|Finland |{1} |Europe |NULL |NULL |
|3 |Berlin |{1,2} |Europe |Germany |NULL |
+--+-----------+-------+--------+--------+--------+