Как я фиксирую Обязательный Отказ в Мыле?

Вы можете использовать выражение case when:

select  jobs.*  
from    (
        select     id 
        from       locations 
        where      name = "Europe"
        union all
        select     child.id 
        from       locations main
        inner join locations child 
                on main.name = case when main.continent is null then child.continent
                                    when main.country   is null then child.country
                                    when main.admin1    is null then child.admin1
                                    when main.admin2    is null then child.admin2
                                                                else child.city
                               end
        where      main.name = "Europe"
        ) sub
inner join jobs 
        on jobs.location_id = sub.id