code: protected function execute(){
          if(!isset($this->rows)){
              $result     = false;
              $exception  = null;
              $parameters = array();
              foreach(array_merge($this->select, array(
                  $this,
                  $this->group,
                  $this->having
              ), $this->order, $this->unionOrder) as $val){
                  if(($val instanceof NotORM_Literal || $val instanceof self) && $val->parameters){
                      $parameters = array_merge($parameters, $val->parameters);
                  }
              }
              try{
                  $result = $this->query($this->__toString(), $parameters);
              }
              catch(PDOException $exception){
                  // handled later
              }
              if(!$result){
                  if(!$this->select && $this->accessed){
                      $this->accessed = '';
                      $this->access   = array();
                      $result         = $this->query($this->__toString(), $parameters);
                  }elseif($exception){
                      throw $exception;
                  }
              }
              $this->rows = array();
              if($result){
                  $result->setFetchMode(PDO::FETCH_ASSOC);
                  foreach($result as $key => $row){
                      if(isset($row[$this->primary])){
                          $key = $row[$this->primary];
                          if(!is_string($this->access)){
                              $this->access[$this->primary] = true;
                          }
                      }
                      //$this->rows[$key] = new $this->notORM->rowClass($row, $this);
                      if ($this->notORM->isKeepPrimaryKeyIndex) {
                          //@dogstar 采用主键作为下标 2015-12-30
                          $this->rows[$key] = $row;
                      } else {
                          //@dogstar 改用返回数组 2014-11-01
                          $this->rows[] = $row;
                      }
                  }
              }
              $this->data = $this->rows;
          }
      }
docs: Execute the built query