1、题目：显示员工数最多的部门信息，显示部门ID、名称、部门员工数，部门的主管经理姓名*/
答案：
select
dept.department_id,dept.department_name,
count(*),e1.first_name||e1.last_name
from
departments dept,employees e ,employees e1
where
dept.department_id=e.department_id
and
dept.manager_id=e1.employee_id
group by
dept.department_id,dept.department_name,
e1.first_name||e1.last_name
having count(*)=(select max(count(*)) from employees group by department_id )
1、<1>/*第一步，查询出员工人数最多的部门的员工人数*/
select max(count(*)) from employees
group by department_id
2、<2>/*第二步，员工表和部门表连接，以部门分组，*/
select
dept.department_id,dept.department_name,
count(*),e1.first_name||e1.last_name
from
departments dept,employees e ,employees e1
where
dept.department_id=e.department_id
and
dept.manager_id=e1.employee_id
group by
dept.department_id,dept.department_name,
e1.first_name||e1.last_name
3、<3> /*第三部，合并一二两部，讲第一步查询出来的结果作为第二部分组完的条件*/
select
dept.department_id,dept.department_name,
count(*),e1.first_name||e1.last_name
from
departments dept,employees e ,employees e1
where
dept.department_id=e.department_id
and
dept.manager_id=e1.employee_id
group by
dept.department_id,dept.department_name,
e1.first_name||e1.last_name
having count(*)=
(select max(count(*)) from employees
group by department_id )
4、在这里提醒一下，由于可能出现两个部门人数最多的部门，所以先查出部门人数最多的人数，再去查询部门
