问在使用PDO的INSRET之后SELECT为空

EN

if (isset($_GET['action'])) {
    if($_GET['action'] == 'add'){
        if (isset($_GET['propid'])) {
            $query = "SELECT id as property_id, name, enumtype, openentries, required FROM properties WHERE id = :prop_id";
            $params = array();
            $params[] = array(':prop_id', $_GET['propid'], 'int');
            $db->query($query, $params);
            $property = $db->fetch();

            // Assign property to categeory
            $query = "INSERT INTO categories_properties(cat_id, property_id, required) VALUES (:cat_id, :prop_id, :req)";
            $params = array();
            $params[] = array(':cat_id', $catid, 'int');
            $params[] = array(':prop_id', $_GET['propid'], 'int');
            $params[] = array(':req', $property['required'], 'int');
            $db->query($query, $params);
            $success = $db->result();
        }
    }elseif($_GET['action'] == 'delete'){
        if (isset($_GET['propid'])) {
            // Assign property to categeory
            $query = "DELETE FROM categories_properties WHERE cat_id = :cat_id AND property_id = :prop_id";
            $params = array();
            $params[] = array(':cat_id', $catid, 'int');
            $params[] = array(':prop_id', $_GET['propid'], 'int');
            $db->query($query, $params);
        }
    }

}


// Get assigned properties
$query = "SELECT cp.cat_id cat_id, cp.property_id property_id, cp.required required, p.name property_name, p.enumtype property_enum, c.cat_name cat_name FROM (categories_properties cp INNER JOIN properties p ON (cp.property_id = p.id)) LEFT OUTER JOIN categories c ON (cp.cat_id = c.cat_id) WHERE cp.cat_id = :cat_id";
$params = array();
$params[] = array(':cat_id', $catid, 'int');
$db->query($query, $params);

$cat_properties = array();
while ($row = $db->fetch()) {
    $cat_properties[] = $row;
}

print_r($cat_properties); // <<< empty if action == add, correct if action == delete or action not set