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 & args_func = ast_function->as<ASTFunction &>();
    if (!args_func.arguments)
        throw Exception("Table function 'mysql' must have arguments.", ErrorCodes::LOGICAL_ERROR);
    ASTs & args = args_func.arguments->children;
    if (args.size() < 5 || args.size() > 7)
        throw Exception("Table function 'mysql' requires 5-7 parameters: MySQL('host:port', database, table, 'user', 'password'[, replace_query, 'on_duplicate_clause']).",
            ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
    for (auto & arg : args)
        arg = evaluateConstantExpressionOrIdentifierAsLiteral(arg, context);
    String host_port = args[0]->as<ASTLiteral &>().value.safeGet<String>();
    remote_database_name = args[1]->as<ASTLiteral &>().value.safeGet<String>();
    remote_table_name = args[2]->as<ASTLiteral &>().value.safeGet<String>();
    user_name = args[3]->as<ASTLiteral &>().value.safeGet<String>();
    password = args[4]->as<ASTLiteral &>().value.safeGet<String>();
    
    size_t max_addresses = context->getSettingsRef().glob_expansion_max_elements;
    auto addresses = parseRemoteDescriptionForExternalDatabase(host_port, max_addresses, 3306);
    pool.emplace(remote_database_name, addresses, user_name, password);
    if (args.size() >= 6)
        replace_query = args[5]->as<ASTLiteral &>().value.safeGet<UInt64>() > 0;
    if (args.size() == 7)
        on_duplicate_clause = args[6]->as<ASTLiteral &>().value.safeGet<String>();
    if (replace_query && !on_duplicate_clause.empty())
        throw Exception(
            "Only one of 'replace_query' and 'on_duplicate_clause' can be specified, or none of them",
            ErrorCodes::BAD_ARGUMENTS);
}
ColumnsDescription TableFunctionMySQL::getActualTableStructure(ContextPtr context) const