public function buildColumns($columns)
    {
        if (!is_array($columns)) {
            if (strpos($columns, '(') !== false) {
                return $columns;
            } else {
                $columns = preg_split('/\s*,\s*/', $columns, -1, PREG_SPLIT_NO_EMPTY);
            }
        }
        foreach ($columns as $i => $column) {
            if ($column instanceof Expression) {
                $columns[$i] = $column->expression;
            } elseif (strpos($column, '(') === false) {
                $columns[$i] = $this->db->quoteColumnName($column);
            }
        }

        return is_array($columns) ? implode(', ', $columns) : $columns;
    }