with RECURSIVE
file_group(id, name, parent_id) as (values (2,'root',null),(3,'1',2), (4,'2',2),(5,'1.1',3),(7,'2.1',4),(8,'2.2',4)),
file (id, name,group_id) as (values (1,'file1',2),(2,'file2',8),(3,'file3',8)),

t1 as (
	select id, name, parent_id 
    
    from 
    (
    	select * from file_group
        union ALL
        select id + (select max (id) from file_group ), name, group_id from file a1 
    )
    as t3 
    
    where parent_id is null

    union ALL

    select t2.id, t2.name, t2.parent_id from     (
    	select * from file_group
        union ALL
        select id + (select max (id) from file_group ), name, group_id from file a1 
    ) as t2 
    
    join t1 on t1.id=t2.parent_id
)
select *  from t1