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?

#include <cerrno>
#include <cstdlib>
#include <Poco/String.h>
#include <IO/ReadBufferFromMemory.h>
#include <IO/ReadHelpers.h>
#include <Parsers/DumpASTNode.h>
#include <Common/typeid_cast.h>
#include <Common/StringUtils/StringUtils.h>
#include <Parsers/ASTAsterisk.h>
#include <Parsers/ASTCollation.h>
#include <Parsers/ASTColumnsTransformers.h>
#include <Parsers/ASTExpressionList.h>
#include <Parsers/ASTFunction.h>
#include <Parsers/ASTFunctionWithKeyValueArguments.h>
#include <Parsers/ASTIdentifier.h>
#include <Parsers/ASTLiteral.h>
#include <Parsers/ASTOrderByElement.h>
#include <Parsers/ASTInterpolateElement.h>
#include <Parsers/ASTQualifiedAsterisk.h>
#include <Parsers/ASTQueryParameter.h>
#include <Parsers/ASTSelectWithUnionQuery.h>
#include <Parsers/ASTSubquery.h>
#include <Parsers/ASTTTLElement.h>
#include <Parsers/ASTWindowDefinition.h>
#include <Parsers/ASTAssignment.h>
#include <Parsers/ASTColumnsMatcher.h>
#include <Parsers/ASTExplainQuery.h>
#include <Parsers/ASTSelectQuery.h>