php多条件查询

$sql="select *from think_kaoqin WHERE(1=1)";      if(!empty($uid))            {                $sql .=" and uid= ".$uid;            }             if(!empty($time1) && empty($time2))            {                                   $sql .=" and riqi >= '".$time1."'";            }            if (!empty($time2) &&empty($time1)) {                           $sql .=" and riqi <= '".$time2."'";            }           if (!empty($time1) && !empty($time2)) {                $sql .=" and riqi >= '".$time1."' and riqi <='".$time2."'";            }  $this->info=M('kaoqin')->query($sql);