Oracle 18c

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CREATE TABLE mytable(
   pk     INTEGER  NOT NULL PRIMARY KEY 
  ,fk     INTEGER 
  ,parent INTEGER  NOT NULL
);

begin
INSERT INTO mytable(pk,fk,parent) VALUES (001,23,000);
INSERT INTO mytable(pk,fk,parent) VALUES (002,NULL,001);
INSERT INTO mytable(pk,fk,parent) VALUES (003,46,001);
INSERT INTO mytable(pk,fk,parent) VALUES (004,12,000);
INSERT INTO mytable(pk,fk,parent) VALUES (005,NULL,004);
INSERT INTO mytable(pk,fk,parent) VALUES (006,NULL,005);
end;
/
1 rows affected

with cte (pk, fk, parent, lvl, pat) as (
    select to_char(pk), fk, parent, 0, to_char(pk) from mytable where parent = 0
    union all
    select lpad(' ', 2 * (lvl + 1)) || t.pk, coalesce(t.fk, c.fk), t.parent, c.lvl + 1, c.pat || '/' || t.pk
    from cte c
    inner join mytable t on t.parent = c.pk
)
select pk, fk, parent from cte order by pat


PK
FK
PARENT


1
23
0

  2
23
1

  3
46
1

4
12
0

  5
12
4

    6
12
5