union all

with recursive tree as (
select id,
    parent_id,
1 as 'level'
from ecoli_data
where parent_id is null

union
    
select e1.id,
    e1.parent_id,
    e2.level + 1
from ecoli_data e1
join tree e2
on e2.id=e1.parent_id   -- 크로스 조인
)

select id
from tree
where level=3
order by id

 

union해줄 때 테이블 구조 일치시키기!