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 as("AS");
    ParserExpression element_p;
    ParserIdentifier ident_p;
    ASTPtr ident;
    if (!ident_p.parse(pos, ident, expected))
        return false;
    ASTPtr expr;
    if (as.ignore(pos, expected))
    {
        if (!element_p.parse(pos, expr, expected))
            return false;
    }
    else
        expr = ident;
    auto elem = std::make_shared<ASTInterpolateElement>();
    elem->column = ident->getColumnName();
    elem->expr = expr;
    elem->children.push_back(expr);
    node = elem;
    return true;
}
bool ParserFunctionWithKeyValueArguments::parseImpl(Pos & pos, ASTPtr & node, Expected & expected)