本文展示了如何使用SQL查询语句来筛选并分组部门，同时限制每个部门的员工数量不超过20人。通过示例查询，读者可以了解如何应用GROUP BY 和 HAVING 子句进行复杂的数据过滤和聚合。
select deptno, count(*)
from employees
group by deptno
having deptno <= 20;
DEPTNO COUNT(*)
-------- --------
10 3
20 5
select deptno, count(*)
from employees
where deptno <= 20
group by deptno;
DEPTNO COUNT(*)
-------- --------
10 3
20 5
from employees
group by deptno
having deptno <= 20;
DEPTNO COUNT(*)
-------- --------
10 3
20 5
select deptno, count(*)
from employees
where deptno <= 20
group by deptno;
DEPTNO COUNT(*)
-------- --------
10 3
20 5
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