(转)php连接mysql如何判断数据为空？

<?php
$result_a=mysql_query("select * from product_tag where product_id=$row[id]");
$num = 0;
$num = mysql_num_rows($result_a);

if(!$num == 0){
?>
<td><div style="padding-left:40px;">Sport：</div></td><td>
<?php
 while($row_a=mysql_fetch_array($result_a))
 {
  $row_b=mysql_fetch_array(mysql_query("SELECT * FROM `tag_tag` WHERE id=$row_a[tag_id]"));
  if($row_b[classid]==5)
  {
   echo $row_b[tag_name];
  }
 }
?>
</td>

<?php
}
?>