How to load a hierarchical table using a sequence in Oracle SQL

If I have a hierarchical table : TREE (ID, PARENT_ID, NAME)

How can this be loaded in sql using a sequence?
In PLSQL we could use variables etc. but I'm wondering if it can be done in SQL.

This obviously won't work, it fails on row 2 because ID = PARENT_ID and I have a check constraint preventing it.

insert into tree (TREE_SEQ.NEXTVAL, NULL, 'root');
insert into tree (TREE_SEQ.CURRVAL, TREE_SEQ.NEXTVAL, 'branch');

This works, but I have concerns about concurrency.
It also assumes that the sequence is defined with INCREMENT BY 1

insert into tree (TREE_SEQ.NEXTVAL, NULL, 'root');
insert into tree (TREE_SEQ.NEXTVAL, TREE_SEQ.CURRVAL - 1, 'branch');

You ought to just use PL/SQL:

DECLARE
  v_id tree.id%TYPE;
BEGIN
  INSERT INTO tree (ID, PARENT_ID, NAME)
  VALUES (TREE_SEQ.NEXTVAL, NULL, 'root')
  RETURNING id INTO v_id;

  INSERT INTO tree (ID, PARENT_ID, NAME)
  VALUES (TREE_SEQ.NEXTVAL, v_id, 'branch');
END;
/

Which, given the setup:

CREATE TABLE TREE (
  ID        NUMBER PRIMARY KEY,
  PARENT_ID REFERENCES tree(id),
  NAME      VARCHAR2(10)
);

CREATE SEQUENCE tree_seq;

Then after the PL/SQL block, the table contains:

However, if you create a function to wrap the sequence:

CREATE FUNCTION get_tree_sequence RETURN TREE.ID%TYPE
IS
BEGIN
  RETURN TREE_SEQ.NEXTVAL;
END;
/

Then you can use it in an SQL statement:

INSERT INTO tree (ID, PARENT_ID, NAME)
SELECT id,
       LAG(id) OVER (ORDER BY ROWNUM),
       CASE ROWNUM WHEN 1 THEN 'root' ELSE 'branch' END
FROM   (
  SELECT get_tree_sequence() AS id
  FROM   DUAL
  CONNECT BY LEVEL <= 2
);

Then the table now contains:

fiddle