什么是mysql查询以显示没有部门的国家总数

RELATION BETWEEN COUNTRIES AND DEPARTMENTS

我试图以不同的方式做到这一点,但这就是我走了多远

select  count(countries.country_name)  from countries  
join locations on countries.country_id = locations.country_id 
left join departments on locations.location_id = departments.location_id 
where department_id is null;