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?

{
    
    if (pos->type == TokenType::QuotedIdentifier)
    {
        ReadBufferFromMemory buf(pos->begin, pos->size());
        String s;
        if (*pos->begin == '`')
            readBackQuotedStringWithSQLStyle(s, buf);
        else
            readDoubleQuotedStringWithSQLStyle(s, buf);
        if (s.empty())    
            return false;
        node = std::make_shared<ASTIdentifier>(s);
        ++pos;
        return true;