Recursively generate JSON tree from hierarchical table in Postgres and jOOQ - json

Question

I have a hierarchical table in Postgres database, e.g. category. The structure is simple like this:
id
parent_id
name
1
null
A
2
null
B
3
1
A1
4
3
A1a
5
3
A1b
6
2
B1
7
2
B2
What i need to get from this table is recursive deep tree structure like this:
[
{
"id": 1,
"name": "A",
"children": [
{
"id": 3,
"name": "A1",
"children": [
{
"id": 4,
"name": "A1a",
"children": []
},
{
"id": 5,
"name": "A1b",
"children": []
}
]
}
]
},
{
"id": 2,
"name": "B",
"children": [
{
"id": 6,
"name": "B1",
"children": []
},
{
"id": 7,
"name": "B2",
"children": []
}
]
},
]
Is it possible with unknown depth using combination of WITH RECURSIVE and json_build_array() or some other solution?

Answer 1

I found an answer to this question in this excellent blog post here, as I was wondering how to generalise over this problem in jOOQ. It would be useful if jOOQ could materialise arbitrary recursive object trees in a generic way: https://github.com/jOOQ/jOOQ/issues/12341
In the meantime, use this SQL statement, which was inspired by the above blog post, with a few modifications. Translate to jOOQ if you must, though you might as well store this as a view:
WITH RECURSIVE
d1 (id, parent_id, name) as (
values
(1, null, 'A'),
(2, null, 'B'),
(3, 1, 'A1'),
(4, 3, 'A1a'),
(5, 3, 'A1b'),
(6, 2, 'B1'),
(7, 2, 'B2')
),
d2 AS (
SELECT d1.*, 0 AS level
FROM d1
WHERE parent_id IS NULL
UNION ALL
SELECT d1.*, d2.level + 1
FROM d1
JOIN d2 ON d2.id = d1.parent_id
),
d3 AS (
SELECT d2.*, jsonb_build_array() children
FROM d2
WHERE level = (SELECT max(level) FROM d2)
UNION (
SELECT (branch_parent).*, jsonb_agg(branch_child)
FROM (
SELECT
branch_parent,
to_jsonb(branch_child) - 'level' - 'parent_id' AS branch_child
FROM d2 branch_parent
JOIN d3 branch_child ON branch_child.parent_id = branch_parent.id
) branch
GROUP BY branch.branch_parent
UNION
SELECT d2.*, jsonb_build_array()
FROM d2
WHERE d2.id NOT IN (
SELECT parent_id FROM d2 WHERE parent_id IS NOT NULL
)
)
)
SELECT jsonb_pretty(jsonb_agg(to_jsonb(d3) - 'level' - 'parent_id')) AS tree
FROM d3
WHERE level = 0;
dbfiddle. Again, read the linked blog post for an explanation of how this works