แสดงรหัสลูกค้าที่ไม่มีบัญชี
Show customer ids who don't have an account.
df_Customers = pd.DataFrame(columns = ['customer_id'])
df_Accounts = pd.DataFrame(columns = ['customer_id'])
df_Customers[~df_Customers['customer_id'].isin(df_Accounts['customer_id'])]['customer_id']