查询各个部门的部门号，部门名，部门人数及部门平均工资

select d.departmentid,
                         departmentname,
                         ds.amount,
                         ds.avgsal
                          from departments d,
                         (select departmentid,
                                 count(*) amount,
                                 avg(salary) avgsal
                            from employees
                           group by departmentid) ds
                         where d.departmentid = ds.departmentid