Question

注意，这是一个家庭作业问题！

显示部门编号，部门名称以及在部门中工作人数最少的员工人数。

我可以弄清楚每个部门有多少员工，但我如何只展示员工人数最少的部门呢？

SELECT employees.department_id, departments.department_name, COUNT(employee_id)
FROM employees
JOIN departments
ON employees.department_id = departments.department_id
GROUP BY employees.department_id, departments.department_name;

Answer 1

您需要的是having子句，它允许您在应用group by子句后应用条件。换句话说，它允许您对聚合表达式应用条件：

SELECT employees.department_id, departments.department_name, COUNT(employee_id)
FROM employees
JOIN departments
ON employees.department_id = departments.department_id
GROUP BY employees.department_id, departments.department_name;
HAVING COUNT(*) > 5 -- Here!

Answer 2

Oracle（以及大多数其他DBMS）支持窗口化聚合函数，这些函数通常比旧式SQL更有效：

SELECT department_id, department_name, CNT
FROM
 (
   SELECT employees.department_id, departments.department_name,  
      COUNT(employee_id) AS CNT, 
      RANK() 
      OVER (ORDER BY COUNT(employee_id) DESC) AS rnk
   FROM employees
   JOIN departments
   ON employees.department_id = departments.department_id
   GROUP BY employees.department_id, departments.department_name
 ) dt
WHERE rnk = 1;

{<1}}在聚合后处理并将1分配给最高COUNT，只需运行内部SQL即可查看实际数据。

ORACLE SQL：显示部门的部门名称，ID和员工数量。拥有最少的员工数量

2 个答案: