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?

{
        expected.add(pos, "substitution name (identifier)");
        return false;
    }
    String name(pos->begin, pos->end);
    ++pos;
    if (pos->type != TokenType::Colon)
    {
        expected.add(pos, "colon between name and type");
        return false;
    }
    ++pos;
    auto old_pos = pos;
    ParserDataType type_parser;
    if (!type_parser.ignore(pos, expected))
    {
        expected.add(pos, "substitution type");
        return false;
    }
    String type(old_pos->begin, pos->begin);
    if (pos->type != TokenType::ClosingCurlyBrace)
    {
        expected.add(pos, "closing curly brace");
        return false;
    }
    ++pos;
    node = std::make_shared<ASTQueryParameter>(name, type);
    return true;
}
bool ParserMySQLGlobalVariable::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)