private function getColumnsString()
    {
        if (count($this->columns) === 1 && $this->columns[0] === '*') {
            return '*';
        }
        $columns = array_map(function ($column) {
            return (strpos($column, '.') === false)? $this->table . '.' . $column : $column;
        }, $this->columns);
        return implode(', ', $columns);
    }