$row = $this->db->get();//得出对象结果集
$result = array();
if($row) {
    //转化为数组
   while($value = $row->fetch_array()) {
            $result[] = $value;
     }
}
print_r($result);eixt;
return $result;

$result=$mysql_query("select `id` from table_a");
$ar=array();
while($rows=$mysql_fetch_array($result))
{
   $ar[] = rows[‘id‘];//把id存放到数组里
}