4 tables, find department with the highest average salary
First query:

select
    e.employee_name as employee
    ,e.street
    ,e.city
    ,mgr.employee_name as manager
    ,mgr.street
    ,mgr.city
from 
    employee e
    join manages mgs on mgs.employee_name = e.employee_name
    join employee mgr on mgr.employee_name = mgs.manager_name
where
    e.street = mgr.street
    and e.city = mgr.city


Second query:

select 
    d.department_name 
    ,avg(salary) as AvgSalary
from 
   department d
   join works w on w.department_name = d.department_name
group by
   d.department_name
order by
   AvgSalary desc