Nested leaf node - sql

Question

I've stored a tree that contains multiple nodes. Every record in that table represents a node and its parent-node, as follows:
node_id
parent_id
A
null
B
A
C
A
D
B
E
B
As a result, the visual tree would look like this: tree-nodes
My goal is to create a function that'll hold the JSON path for every leaf in the tree. So for my current table, the result should behave as shown below:
leaf_id
json_path
C
{"name": "A", "children": [{ "name": "C", "children": [] }] }
D
{"name": "A", "children": [{ "name": "B", "children": [{ "name": "D", "children": [] }] }] }
E
{"name": "A", "children": [{ "name": "B", "children": [{ "name": "E", "children": [] }] }] }
There's already a question with a function that does the format I'm trying to achieve (link below):
nested-json-object.
However, the written function selects the entire tree. Therefore, as I mentioned above, I need the path of every leaf node.

Answer 1

Using a recursive cte:
with recursive cte(id, l, js) as (
select t.node_id, t.node_id, jsonb_build_object('name', t.node_id, 'children', '[]'::jsonb) from tbl t
where not exists (select 1 from tbl t1 where t1.parent_id = t.node_id)
union all
select t3.parent_id, t3.l, jsonb_build_object('name', t3.parent_id, 'children', t3.js)
from (select t2.parent_id, t2.l, jsonb_agg(t2.js) js
from (select t1.parent_id, c.l, c.js from tbl t1
join cte c on c.id = t1.node_id) t2 group by t2.parent_id, t2.l) t3
)
select l leaf_id, v json_path from cte
cross join jsonb_array_elements(js -> 'children') v where id is null
See fiddle