public function select($columns= '*') {
        if (!is_array($columns)) {
            $columns= trim($columns);
            if ($columns == '*' || $columns === $this->_alias . '.*' || $columns === $this->xpdo->escape($this->_alias) . '.*') {
                $columns= $this->xpdo->getSelectColumns($this->_class, $this->_alias, $this->_alias . '_');
            }
            $columns= explode(',', $columns);
            foreach ($columns as $colKey => $column) $columns[$colKey] = trim($column);
        }
        if (is_array ($columns)) {
            if (!is_array($this->query['columns'])) {
                $this->query['columns']= $columns;
            } else {
                $this->query['columns']= array_merge($this->query['columns'], $columns);
            }
        }
        return $this;
    }