Pros:
- it is convenient to type and read numbers this way;
- Ada, Perl, C++, Go and Rust already support that;
- can support thousands as well as lakhs and crores;

Cons:
- standard SQL does not have this support;
- PostgreSQL treat `SELECT 1_000_000` as number one with `_000_000` alias: http://sqlfiddle.com/#!17/0a28f/400
- C++ has `1'000'000` instead of `1_000_000`;

What do you think?

{
public:
    bool parse(IParser::Pos & pos, Collections & collections, ASTPtr & node, Expected & expected) override;
private:
    Container container;
};
class MapCollection : public ICollection
{
public:
    bool parse(IParser::Pos & pos, Collections & collections, ASTPtr & node, Expected & expected) override;
private:
    Map container;
};
bool parseAllCollectionsStart(IParser::Pos & pos, Collections & collections, Expected & )
{
    if (pos->type == TokenType::OpeningCurlyBrace)
        collections.push_back(std::make_unique<MapCollection>());
    else if (pos->type == TokenType::OpeningRoundBracket)
        collections.push_back(std::make_unique<CommonCollection<Tuple, TokenType::ClosingRoundBracket>>());
    else if (pos->type == TokenType::OpeningSquareBracket)
        collections.push_back(std::make_unique<CommonCollection<Array, TokenType::ClosingSquareBracket>>());
    else
        return false;
    ++pos;
    return true;
}
template <class Container, TokenType end_token>
bool CommonCollection<Container, end_token>::parse(IParser::Pos & pos, Collections & collections, ASTPtr & node, Expected & expected)