Counting number of visitors who did NOT log in
Do it with a LEFT JOIN

SELECT Count(DISTINCT accessid) AS c
FROM   accesslog a
LEFT JOIN
(SELECT accessid
 FROM   accesslog
 WHERE  DATE = '2014-01-21'
 AND userid &gt; 0
 AND is_robot = 0
 AND is_admin = 0 
 GROUP BY accessid)x
ON a.accessid=x.accessid
WHERE  a.date = '2014-01-21'
   AND a.userid = 0
   AND a.is_robot = 0
   AND a.is_admin = 0
   AND a.url = 'www.example.com/'
   AND x.accessid IS NULL