I can't immediatly say that I know how you could do this by putting the query in an if statement.
However, there are several ways to solve this problem.
Method #1: Send a cookie to the browser named something like 'readArticle12' where 12 would be an ID for a specific article. You can then test if the person has accessed this article before (set a two day timeout or so on the cookie).. See php manual on setcookie().
Another solution would be to, once a person actually reads the article, insert data to a table in your database. The data would include at least the users IP address, and maybe his browser version, and ofcourse an identfier for the article being read. You could then, upon a new load, check this table for entries newer than say two hours from that IP with that browser. If one exists, you should not update your counter.