Adhoc SQL request - response (2) by muthm

This could do the job:

select level0.level0, level1.level1, level2.level2, level3.level3
from (
    select
        row_number() over ( order by level0 ) as rownum,
        level0
    from dx
    where level0 is not null and level1 is null
) level0
full outer join (
    select
        row_number() over ( order by level1 ) as rownum,
        level1
    from dx
    where level1 is not null and level2 is null
) level1
    on level1.rownum = level0.rownum
full outer join (
    select
        row_number() over ( order by level1 ) as rownum,
        level2
    from dx
    where level2 is not null and level3 is null
) level2
    on level2.rownum = coalesce( level0.rownum, level1.rownum )
full outer join (
    select
        row_number() over ( order by level1 ) as rownum,
        level3
    from dx
    where level3 is not null
) level3
    on level3.rownum = coalesce( level0.rownum, level1.rownum, level2.rownum )
order by coalesce( level0.rownum, level1.rownum, level2.rownum, level3.rownum );