แสดงจำนวนลูกค้าทั้งหมดที่ไม่มีบัญชี
Show the number of all customers without an account.
df_customers = pd.DataFrame(columns=['customer_id']);df_accounts = pd.DataFrame(columns=['customer_id'])
len(df_customers[~df_customers['customer_id'].isin(df_accounts['customer_id'])])