Find all associations as a list within postgres adjacency table - postgresql

Question

Given the following:
CREATE TABLE example (
example_name varchar(50),
example_parent varchar(50)
);
insert into example (example_name, example_parent)
values
('e1', NULL),
('e2', 'e1'),
('e3', NULL),
('e4', NULL),
('e5', NULL),
('e6', 'e5'),
('e7', 'e6'),
('e8', 'e7'),
('e9', NULL)
;
Which looks as:
| example_name | example_parent |
|:---------------|:-----------------|
| e1 | |
| e2 | e1 |
| e3 | |
| e4 | |
| e5 | |
| e6 | e5 |
| e7 | e6 |
| e8 | e7 |
| e9 | |
I would like to be able to find all associated values for a particular example_name.
Some examples of what I'd like to have returned for different example_name's:
given e1:
| example_name | associations |
|:---------------|:---------------|
| e1 | e2 |
given e8:
| example_name | associations |
|:---------------|:---------------|
| e8 | e7, e6, e5 |
given e9:
| example_name | associations |
|:---------------|:---------------|
| e9 | |

Answer 1

Use a recursive CTE to get the list for the given value(s). Then use string_agg() function in the main query. (see demo)
with recursive hier_examples(name,parent, level) as
( select example_name, example_parent, 0
from example
where example_name in (<1 or more comma separated values here>)
union all
select he.name, ex.example_parent, level+1
from hier_examples he
join example ex
on (ex.example_name = he.parent)
) --select * from hier_examples;
select name, string_agg(parent, ', ' order by level) associations
from hier_examples
group by name
order by name;