แสดงชื่อแผนกและจำนวนพนักงานในแต่ละแผนก
display the department name and number of employees in each of the department.
departments = pd.DataFrame(columns=['department_name', 'department_id']); employees = pd.DataFrame(columns=['department_id'])
departments.merge(employees.groupby('department_id').size(), left_on = 'department_id', right_index = True)[['department_name', 'size']]