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