我遵循了https://makandracards.com/makandra/39899-install-mysql-5-6-in-ubuntu-16-04本指南，指出了我使用apt update的位置。
我需要mysql 5.6服务器和客户端工具，因为我使用的软件无法与最新的软件配合使用...
W: GPG error: http://repo.mysql.com/apt//ubuntu wily InRelease: The following si gnatures were invalid: EXPKEYSIG 8C718D3B5072E1F5 MySQL Release Engineering <mys ql-build@oss.oracle.com> A4A9406876FCBD3C456770C88C718D3B5072E1F5
E: The repository 'http://repo.mysql.com/apt//ubuntu wily InRelease' is not sign ed.
N: Updating from such a repository can't be done securely, and is therefore disa bled by default.
N: See apt-secure(8) manpage for repository creation and user configuration deta ils.
如何安全地思考不能禁用？