private function createWhereExpression(array $where = [])
    {
        $criteria = [];
        foreach ($where as $index => $value) {
            if (is_int($index)) {
                $criteria[] = $value;
            } else {
                $criteria[] = $this->quote()->into($index, $value);
            }
        }
        return implode(' AND ', $criteria);
    }