Create test table:
`CREATE TABLE test (`id` Int32) ENGINE = MySQL('localhost:3306', '', 'test', 'user', 'pass')`

Attempt insert:
`INSERT INTO test VALUES(1)`

CH sending SQL:
`INSERT INTO ``.`test` (`id`) VALUES (1)`

"``." - sending empty database name in generated SQL

However SELECT statement working with empty database name correctrly

P.S.
This can be useful when working with databases such as sphinx
{
    const auto & settings = context->getSettingsRef();
    const auto tables_and_columns = fetchTablesColumnsList(*pool, remote_database_name, {remote_table_name}, settings, settings.mysql_datatypes_support_level);
    const auto columns = tables_and_columns.find(remote_table_name);
    if (columns == tables_and_columns.end())
        throw Exception("MySQL table " + backQuoteIfNeed(remote_database_name) + "." + backQuoteIfNeed(remote_table_name) + " doesn't exist.", ErrorCodes::UNKNOWN_TABLE);
    return ColumnsDescription{columns->second};
}
StoragePtr TableFunctionMySQL::executeImpl(const ASTPtr & , ContextPtr context, const std::string & table_name, ColumnsDescription ) const