error after update tables of database

<?php
$changes = 0; $error = 0;
$result = dbquery("SHOW COLUMNS FROM ".$db_prefix."site_links LIKE 'link_parent'");
if (dbrows($result) == 0) {
   $result = dbquery("ALTER TABLE ".$db_prefix."site_links ADD link_parent SMALLINT(5) UNSIGNED NOT NULL DEFAULT '0' AFTER link_id");
   if ($result) {
      $result = dbquery("INSERT INTO ".$db_prefix."site_links
         (link_parent, link_name, link_url, link_visibility, link_position, link_window, link_order)
         VALUES ('0', 'Home', 'icon_home.gif', '0', '1', '0', '1')"
      );
      $parent = mysql_insert_id();
      $result = dbquery("UPDATE ".$db_prefix."site_links SET link_parent='$parent' WHERE link_id!='$parent'");
      $changes = 1;
   } else {
      echo "<p align='center'><font color='red'><b>ERROR:</b></font> The link_parent column was not added.</p>";
      $error ++;
   }
} else { $changes = 1; }
if ($changes != 0) {
   $result = dbquery("SHOW COLUMNS FROM ".$db_prefix."site_links LIKE 'link_accessibility'");
   if (dbrows($result) == 0) {
      $result = dbquery("ALTER TABLE ".$db_prefix."site_links ADD link_accessibility TINYINT(3) UNSIGNED NOT NULL DEFAULT '0' AFTER link_visibility");
      if ($result) {
         $result = dbquery("UPDATE ".$db_prefix."site_links SET link_accessibility=link_visibility");
      } else {
         echo "<p align='center'><font color='red'><b>ERROR:</b></font> The link_accessibility column was not added.</p>";
         $error ++;
      }
   }
}
$result = dbquery("SHOW COLUMNS FROM ".$db_prefix."site_links LIKE 'link_collapse'");
if (dbrows($result) == 0) {
   $result = dbquery("ALTER TABLE ".$db_prefix."site_links ADD link_collapse TINYINT(1) UNSIGNED NOT NULL DEFAULT '0' AFTER link_accessibility");
   if (!$result) {
      echo "<p align='center'><font color='red'><b>ERROR:</b></font> The link_collapse column was not added.</p>";
      $error ++;
   }
}
if ($error == 0) { echo "<p align='center'>Table alterations are complete.</p>"; }
?>