/*17. Write a query to find all the salesmen who worked for only one customer.*/
SELECT *   
FROM salesman   
WHERE salesman_id IN (  
   SELECT DISTINCT salesman_id   
   FROM customer a   
   WHERE NOT EXISTS (  
      SELECT * FROM customer b   
      WHERE a.salesman_id=b.salesman_id   
      AND a.cust_name<>b.cust_name));