check my php/sql code pls....

[qpurser]

Just started to play around with php and sql stuff but don't feel that comfortable with it yet.
I am trying to create a custom page with a member list on the fly. Our member list changes almost every 2 weeks and I wanted my users to be able to get an actual member list without me having to create one every time and post it somewhere.
I tried my coding as below out and works pretty well.
However I wanted to be sure I didn't forget anything that mitt use a lot of server resources. For example do I have to clear the sql request after the page is created? If so how do I do that.

Also I would like to add the "join date" field next to the username. When I just put in "join date" I get just that numeric number from the DB. How do I convert it to a readable date?

Thanks for any help or suggestions

<table width="100%" cellspacing="3" cellpadding="20">
<center><b><u><font color=#855b63 size=6>Memberlist <center></b></u></font>
<br />
<br />

<?php



$result_q = $vbulletin->db->query_read("SELECT username, joindate FROM user
									   WHERE user.usergroupid <> '12' AND user.usergroupid = '9' AND user.membergroupids <> '13'
   AND user.membergroupids NOT LIKE '%12%' ORDER 
    BY user.username");


if ($vbulletin->db->num_rows($result_q) >0)

{
	
     $i = 0;
    $max_columns = 4;
	
     while ($result_r = $vbulletin->db->fetch_array($result_q))
	
    {
		
	    // make the variables easy to deal with
      // extract($row);
	

       // open row if counter is zero
       if($i == 0)
          echo "<tr>";	
		  if($result_q != "" && $result_q != null)
		  
          echo "<td><center><font color=#855b63 size=3>$result_r[username] <center></font></td>";
    
       // increment counter - if counter = max columns, reset counter and close row
       if(++$i == $max_columns) 
       {
           echo "</tr>";
           $i=0;
       }  // end if 
   } // end while
} // end if results

// clean up table - makes your code valid!
if($i < $max_columns)
{
    for($j=$i; $j<$max_columns;$j++)
        echo "<td>&nbsp;</td>";
}
 ?>
</tr>
<form><input type="button" value=" Print this page "
onclick="window.print();return false;" /></form>
</table>