查询人数最多的那个部门的部门id?

1.先求出每个部门的人数

select department_id,count(*) from employees e group by department_id;

2.在求出每个部门人数的最大值

select max(c) from (select department_id,count(*) c from employees group by department_id ) e;

3.求哪个部门的人数，等于上面求的的最大值

select department_id,count(*) from employees group by department_id having count(*)=

(

select max(c) from (select department_id,count(*) c from employees group by department_id) e

);