แสดงจำนวนลูกค้าทั้งหมดที่ไม่มีบัญชี ไม่จำเป็นต้องใช้ทุกตาราง
Show the number of all customers without an account.It is not neccessary to use all the tables.
 customers:  customer_id [ number]  customer_first_name [ text]  customer_last_name [ text]  customer_address [ text]  customer_phone [ text]  customer_email [ text]  other_customer_details [ text] 
SELECT COUNT(*) FROM customers WHERE NOT customer_id IN (SELECT customer_id FROM accounts)