WHERE department_id IN (SELECT department_id FROM departments WHERE department_name = 'Sales') 如何筛选员工？
WHERE department_id IN (SELECT department_id FROM departments WHERE department_name = 'Sales')
内层查询返回匹配的部门 ID，外层查询保留 ID 属于该集合的员工
外层查询直接将员工姓名与单词 Sales 进行比较
内层查询将每名员工的部门重命名为 Sales
该表达式对员工进行分组，并统计每个部门的数量