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?

{
    ParserKeyword s_as("AS");
    ParserIdentifier id_p;
    bool has_as_word = s_as.ignore(pos, expected);
    if (!allow_alias_without_as_keyword && !has_as_word)
        return false;
    if (!id_p.parse(pos, node, expected))
        return false;
    if (!has_as_word)
    {
        /** In this case, the alias can not match the keyword -
          *  so that in the query "SELECT x FROM t", the word FROM was not considered an alias,
          *  and in the query "SELECT x FR FROM t", the word FR was considered an alias.
          */
        const String name = getIdentifierName(node);
        for (const char ** keyword = restricted_keywords; *keyword != nullptr; ++keyword)