MySQL 8.0

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create table Tree(
id int ,
p_id int 
);

insert into Tree values
(1,null),
(2,1),
(3,1),
(4,2),
(5,2);
select * from Tree;

id
p_id

1
null

2
1

3
1

4
2

5
2

SELECT id FROM Tree
WHERE id IN (SELECT p_id FROM Tree) ;

id

1

2

SELECT id FROM Tree
WHERE  NOT exists (SELECT 1 FROM Tree t1 where t1.p_id=Tree.id) ;

id

3

4

5

id	p_id
1	null
2	1
3	1
4	2
5	2