function column($name, $type, $len = 4)
    {
        parent::column($name, $type, $len);
        return $this;
    }
function column($name, $type, $len = 4)
    {
        parent::column($name, $type, $len);
        return $this;
    }
增加字段定义
@param     $name
@param     $type
@param int $len