MySQL 8.0

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select version();

version()

8.0.27

create table users(id int , parent_id int);
create table mlm(id int);

insert into users(id, parent_id) values (2,1), (3,1),(4,1),
(5,2),(6,2);
insert into mlm(id) values (4);
select * from users; select * from mlm;

id
parent_id

2
1

3
1

4
1

5
2

6
2

id

4

SELECT u.id
FROM users u
JOIN users uc ON uc.parent_id = u.id
WHERE u.parent_id = 1
UNION
SELECT u.id
FROM users u
JOIN mlm m USING(id)
WHERE u.parent_id = 1

id

2

4