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?

&& ParserDataType().parse(pos, type_ast, expected))
    {
        String s;
        size_t data_size = data_end - data_begin;
        if (is_string_literal)
        {
            ReadBufferFromMemory buf(data_begin, data_size);
            readQuotedStringWithSQLStyle(s, buf);
            assert(buf.count() == data_size);
        }
        else
            s = String(data_begin, data_size);
        auto literal = std::make_shared<ASTLiteral>(std::move(s));
        node = createFunctionCast(literal, type_ast);
        return true;
    }
    return false;
}
bool ParserNull::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)
{
    ParserKeyword nested_parser("NULL");
    if (nested_parser.parse(pos, node, expected))
    {
        node = std::make_shared<ASTLiteral>(Null());
        return true;
    }
    else
        return false;
}
bool ParserBool::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)