Get parent id from level with Oracle SQL - sql

Question

I have a hierarchical structure defined by level and order of elements. Is it possible to create "parent_id" column with Oracle SQL without using procedures?
I need to generate red values:
test data:
with t as
(
select 1 id, 'element1' name, 1 level_ from dual union all
select 2 id, 'element2' name, 2 level_ from dual union all
select 3 id, 'element3' name, 3 level_ from dual union all
select 4 id, 'element4' name, 3 level_ from dual union all
select 5 id, 'element5' name, 3 level_ from dual union all
select 6 id, 'element6' name, 3 level_ from dual union all
select 7 id, 'element7' name, 2 level_ from dual union all
select 8 id, 'element8' name, 3 level_ from dual union all
select 9 id, 'element9' name, 4 level_ from dual union all
select 10 id, 'element10' name, 4 level_ from dual union all
select 11 id, 'element11' name, 1 level_ from dual union all
select 12 id, 'element12' name, 2 level_ from dual union all
select 13 id, 'element13' name, 3 level_ from dual union all
select 14 id, 'element14' name, 4 level_ from dual union all
select 15 id, 'element15' name, 4 level_ from dual union all
select 16 id, 'element16' name, 3 level_ from dual union all
select 17 id, 'element17' name, 4 level_ from dual union all
select 18 id, 'element18' name, 4 level_ from dual union all
select 19 id, 'element19' name, 1 level_ from dual
)
select * from t

Answer 1

From Oracle 12, you can use MATCH_RECOGNIZE:
select *
from t
MATCH_RECOGNIZE (
ORDER BY id DESC
MEASURES
child.id AS id,
child.name AS name,
child.lvl AS lvl,
parent.id AS parent_id
ONE ROW PER MATCH
AFTER MATCH SKIP TO NEXT ROW
PATTERN (child ancestors*? (parent | $))
DEFINE
parent AS lvl = child.lvl - 1
)
ORDER BY id
Or, again from Oracle 12, a LATERAL join:
select *
from t c
LEFT OUTER JOIN LATERAL(
SELECT p.id AS parent_id
FROM t p
WHERE c.id > p.id
AND c.lvl = p.lvl + 1
ORDER BY id DESC
FETCH FIRST ROW ONLY
)
ON (1 = 1)
ORDER BY id
Or, in earlier versions:
SELECT id, name, lvl, parent_id
FROM (
SELECT c.*,
p.id AS parent_id,
ROW_NUMBER() OVER (PARTITION BY c.id ORDER BY p.id DESC) AS rn
FROM t c
LEFT OUTER JOIN t p
ON (c.id > p.id AND c.lvl = p.lvl + 1)
)
WHERE rn = 1
ORDER BY id
Which, for the sample data:
CREATE TABLE t (id, name, lvl ) as
select 1, 'element1', 1 from dual union all
select 2, 'element2', 2 from dual union all
select 3, 'element3', 3 from dual union all
select 4, 'element4', 3 from dual union all
select 5, 'element5', 3 from dual union all
select 6, 'element6', 3 from dual union all
select 7, 'element7', 2 from dual union all
select 8, 'element8', 3 from dual union all
select 9, 'element9', 4 from dual union all
select 10, 'element10', 4 from dual union all
select 11, 'element11', 1 from dual union all
select 12, 'element12', 2 from dual union all
select 13, 'element13', 3 from dual union all
select 14, 'element14', 4 from dual union all
select 15, 'element15', 4 from dual union all
select 16, 'element16', 3 from dual union all
select 17, 'element17', 4 from dual union all
select 18, 'element18', 4 from dual union all
select 19, 'element19', 1 from dual;
All output:
ID
NAME
LVL
PARENT_ID
1
element1
1
null
2
element2
2
1
3
element3
3
2
4
element4
3
2
5
element5
3
2
6
element6
3
2
7
element7
2
1
8
element8
3
7
9
element9
4
8
10
element10
4
8
11
element11
1
null
12
element12
2
11
13
element13
3
12
14
element14
4
13
15
element15
4
13
16
element16
3
12
17
element17
4
16
18
element18
4
16
19
element19
1
null
db<>fiddle here