title: How to check if a data exist in database in PHP? (MULTIPLE)
body: <p>I have a registeration form with the if statement:</p>
  
  <pre><code>$query = "SELECT `username` FROM `users` WHERE `username`='$username'";
  $query_run = mysql_query($query);
  
  if (mysql_num_rows($query_run)==1) 
  {
      echo "Username already exist.";
  }
  </code></pre>
  
  <p>The thing i want to do is checking also the "email" and "nickname" if they exist in database.
  How can i do that?</p>