recursive query to find the root parent in oracle - sql

Question

I am trying to figure out the root parent in a table with hierarchical data. The following example works as expected but I need to do something extra. I want to avoid the query to ignore null id1 and show the (root parent - 1) if the root parent is null.
with table_a ( id1, child_id ) as (
select null, 1 from dual union all
select 1, 2 from dual union all
select 2, 3 from dual union all
select 3, NULL from dual union all
select 4, NULL from dual union all
select 5, 6 from dual union all
select 6, 7 from dual union all
select 7, 8 from dual union all
select 8, NULL from dual
)
select connect_by_root id1 as id, id1 as root_parent_id
from table_a
where connect_by_isleaf = 1
connect by child_id = prior id1
order by id 1
This brings up the following data
4 4
6 5
7 5
8 5
5 5
3 null
null null
2 null
1 null
what I want is
3 1
1 1
2 1
4 4
7 5
8 5
5 5
6 5
is it possible?
Thanks for the help

Answer 1

Using a recursive CTE you can do:
with table_a ( id1, child_id ) as (
select null, 1 from dual union all
select 1, 2 from dual union all
select 2, 3 from dual union all
select 3, NULL from dual union all
select 4, NULL from dual union all
select 5, 6 from dual union all
select 6, 7 from dual union all
select 7, 8 from dual union all
select 8, NULL from dual
),
n (s, e) as (
select id1 as s, child_id as e from table_a where id1 not in
(select child_id from table_a
where id1 is not null and child_id is not null)
union all
select n.s, a.child_id
from n
join table_a a on a.id1 = n.e
)
select
coalesce(e, s) as c, s
from n
order by s
Result:
C S
- -
3 1
1 1
2 1
4 4
5 5
7 5
8 5
6 5
As a side note, "Recursive CTEs" are more flexible than the old-school CONNECT BY.

Answer 2

This looks like it works but it may be incorrect, as I do not quite understand the logic behind choosing 1 for this, looks arbitrary to me, not much like real data will be.
As Hogan has asked already, it would be helpful if you could perhaps provide an explanation or an expanded data set to test this hierarchy.
with table_a ( id1, child_id ) as (
select null, 1 from dual union all
select 1, 2 from dual union all
select 2, 3 from dual union all
select 3, NULL from dual union all
select 4, NULL from dual union all
select 5, 6 from dual union all
select 6, 7 from dual union all
select 7, 8 from dual union all
select 8, NULL from dual
)
select connect_by_root id1 as id, id1 as root_parent_id
from table_a
where connect_by_isleaf = 1 and connect_by_root id1 is not null
connect by nocycle child_id = prior nvl(id1, 1)
order by 2, 1;
Sample execution:
FSITJA#dbd01 2019-07-19 13:51:13> with table_a ( id1, child_id ) as (
2 select null, 1 from dual union all
3 select 1, 2 from dual union all
4 select 2, 3 from dual union all
5 select 3, NULL from dual union all
6 select 4, NULL from dual union all
7 select 5, 6 from dual union all
8 select 6, 7 from dual union all
9 select 7, 8 from dual union all
10 select 8, NULL from dual
11 )
12 select connect_by_root id1 as id, id1 as root_parent_id
13 from table_a
14 where connect_by_isleaf = 1 and connect_by_root id1 is not null
15 connect by nocycle child_id = prior nvl(id1, 1)
16 order by 2, 1;
ID ROOT_PARENT_ID
---------- --------------
1 1
2 1
3 1
4 4
5 5
6 5
7 5
8 5
8 rows selected.