30 - 每位经理的下属员工数量

-- 根据reports_to ，获取employee_id,即分组用e1.reports_to，查询用e2.employee_id,e2.name

select 
    e2.employee_id,e2.name ,count(e1.reports_to) reports_count,round(avg(e1.age),0) average_age
from 
    Employees e1 left join Employees e2
on
    e1.reports_to = e2.employee_id
where
    e1.reports_to is not null
group by 
    e1.reports_to;