Ubuntu 20.04 直接 apt 安装的 mysql 是 8.0 ，现在需要安装 5.7 版本，还颇费周章！按照文档直接点进去那个 MySQL APT Repository 中（https://dev.mysql.com/downloads/repo/apt/）只显示了 8.0 ，没有其他版本的 Repository !

查找 mysql 5.7 的 Repository

如果需要找到 5.7 的 Repository ，还得自己去官网一步一步的进来

1. 从官网进到 download 页面后，不要点这个 MySQL APT Repository ，而是要点最右下的那个 Download Archievs

2. 下一步，点 Mysql Community Server

3. 这样就可以看到版本选择的页面了！选择咱需要的 5.7.42， OK！

这里竟然只有 ubuntu 18.04 ，没有 ubuntu 20.04 ？

安装

1. 直接复制上图所示的这个 DEB Bundle 的 链接地址

2. 因为下载了 bundle ，里面包含了mysql server 和其他工具的 .deb包，所以，单独建一个目录比较好

   $ mkdir -p mysql-5.7

   $ cd mysql-5.7/

   ~/mysql-5.7$ wget https://downloads.mysql.com/archives/get/p/23/file/mysql-server_5.7.42-1ubuntu18.04_amd64.deb-bundle.tar

   $ tar xvf mysql-server_5.7.42-1ubuntu18.04_amd64.deb-bundle.tar

    libmysqlclient20_5.7.42-1ubuntu18.04_amd64.deb
    libmysqlclient-dev_5.7.42-1ubuntu18.04_amd64.deb
    libmysqld-dev_5.7.42-1ubuntu18.04_amd64.deb
    mysql-client_5.7.42-1ubuntu18.04_amd64.deb
    mysql-common_5.7.42-1ubuntu18.04_amd64.deb
    mysql-community-client_5.7.42-1ubuntu18.04_amd64.deb
    mysql-community-server_5.7.42-1ubuntu18.04_amd64.deb
    mysql-community-source_5.7.42-1ubuntu18.04_amd64.deb
    mysql-community-test_5.7.42-1ubuntu18.04_amd64.deb
    mysql-server_5.7.42-1ubuntu18.04_amd64.deb
    mysql-testsuite_5.7.42-1ubuntu18.04_amd64.deb
   

3. 现在就可以安装了！

   过程中，可能会遇到 mysql-client : Depends: mysql-community-client (= 5.7.42-1ubuntu18.04) but it is not installable 之类的错误，根据我的经验，干脆反过来，直接安装 mysql-server_5.7.42-1ubuntu18.04_amd64.deb ，然后，按照错误提示一个一个的补充完整！、

$ sudo apt-get install ./mysql-server_5.7.42-1ubuntu18.04_amd64.deb

...
The following packages have unmet dependencies:
 mysql-server : Depends: mysql-community-server (= 5.7.42-1ubuntu18.04) but it is not installable
E: Unable to correct problems, you have held broken packages.

…

修改 root 密码和 host

mysql> use mysql

mysql> update user set host = '%' where user = 'root';

直接修改密码不行？
mysql> SET PASSWORD FOR 'root'@'%' = PASSWORD('XXXXXXXX');
ERROR 1133 (42000): Can't find any matching row in the user table

mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.11 sec)

mysql> SET PASSWORD FOR 'root'@'%' = PASSWORD('XXXXXXXX');
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> select user,host from user;
+---------------+-----------+
| user          | host      |
+---------------+-----------+
| root          | %         |
| mysql.session | localhost |
| mysql.sys     | localhost |
+---------------+-----------+
3 rows in set (0.00 sec)

mysql> FLUSH PRIVILEGES;
Query OK, 0 rows affected (0.11 sec)