PL / SQL-Oracle 9i

SELECT COUNT(DISTINCT cust#)
  FROM customer a, car b
 WHERE a.transaction# = b.transaction#
   AND b.model# = \'HONDA\'
     AND NOT EXISTS 
     (
        SELECT 1
          FROM customer c, car d
         WHERE c.transaction# = d.transaction#
             AND d.model# = \'NISSAN\'
             AND c.cust# = a.cust#
     )

SELECT COUNT(DISTINCT cust.CUST#) AS COUNT FROM CUSTOMER cust INNER JOIN CAR car ON  
cust.TRANS#=car.TRANS# WHERE CAR_MODEL#=\'HONDA\'
AND NOT EXISTS 
(SELECT COUNT(1) FROM CUSTOMER inner_cust INNER JOIN CAR inner_car ON 
inner_cust.TRANS#=inner_car.TRANS# 
AND inner_cust.CUST#=inner_car.CUST# WHERE inner_car.CAR_MODEL#=\'NISSAN\')