Get all of the low-level children of a node - sql

Question

I am struggling with the writing of a query for PostgreSQL. The table resembles a tree and every item can have n children.
I would like to get only and all the children of the lowest level(s) (which don't have any children by themselves) of a given element of the tree.
The structure:
CREATE TABLE items
(
ikey integer NOT NULL,
description character varying(255),
parent integer,
CONSTRAINT i_pk PRIMARY KEY (ikey),
CONSTRAINT i_relation FOREIGN KEY (parent)
REFERENCES items (ikey) MATCH SIMPLE
)
Some values of the table would look like this:
1 "Products" NULL --Parent
2 "Metal" 1
3 "Nails" 2
4 "Chains" 2
5 "Bicycle Chains" 4
6 "Shimano Bicycle Chains" 5
7 "Shimano Bicycle Chains" 5
8 "7mm chain, black" 4
9 "Wood" 1
10 "Cutting Boards" 8
11 "Cutting Board Holder" 8
Most of the solutions on SO deal with not very deep trees which have just 1-2 levels. Or it is known from which parent the children are required.
I would like to select all children of "Chains" (4), which would give the following result:
6 "Shimano Bicycle Chains" 5
7 "Shimano Bicycle Chains" 5
8 "7mm chain, black" 4
To be honest, recursive queries are not my strongest skill. I had the idea already to search upside down - getting all items first which are never used as parents and then going down, but this only shifts the problem to the point again that I have to go down recursively from the given parent which seems a bit over the top.

Answer 1

If you want to find all children items of an item in all levels you can write a recursive CTE. For example:
with recursive
n as (
select * from items where parent = 4 -- Children of "Chains"
union all
select i.*
from n
join items i on i.parent = n.ikey
)
select * from n
Result:
ikey description parent
----- ------------------------- ------
5 Bicycle Chains 4
8 7mm Chain Black 4
6 Shimano Bicycle Chains 1 5
7 Shimano Bicycle Chains 2 5
See running example at DB Fiddle.

Answer 2

You do not need a recursive query. Instead, you can simply select the rows with ikeys that do not occur as parents:
select i1.* from items i join items i1 on i.ikey = i1.parent where i.parent = 4 and
not exists (select 1 from items i2 where i2.parent = i1.ikey)

Answer 3

Another recursive CTE, with extras.
The base key that started the recursion & level of the tree.
WITH RECURSIVE RCTE_OFFSPRING AS (
SELECT ikey as base, 0 as lvl
, ikey, description, parent
FROM items
WHERE ikey = 4
UNION ALL
SELECT cte.base, cte.lvl + 1
, itm.ikey, itm.description, itm.parent
FROM items itm
JOIN RCTE_OFFSPRING cte
ON cte.ikey = itm.parent
)
SELECT *
FROM RCTE_OFFSPRING
WHERE lvl > 0
ORDER BY base, lvl, ikey
base
lvl
ikey
description
parent
4
1
5
Bicycle Chains
4
4
1
8
7mm chain, black
4
4
2
6
Shimano Bicycle Chains
5
4
2
7
Shimano Bicycle Chains
5
db<>fiddle here