private function buildSelectColumns()
    {
        $columns = array();

        foreach ($this->query['select'] as $column => $as) {
            // Normal column select
            if (is_numeric($column)) {
                $columns[] = $this->columnName($as);
            }
            // Alias
            else {
                $columns[] = $this->columnName($column) . " AS `{$as}`";
            }
        }

        // Join columns and return
        return implode(', ', $columns);
    }