好吧，我也遇到过同样的问题，但我已经解决了。由于某种原因，mysql_secure_installation脚本在使用Homebrew安装mysql时不能开箱即用，所以我手动安装了。在CLI中输入:

mysql -u root

这样你就可以进入mysql了。现在执行以下操作(取自mysql_secure_installation):

UPDATE mysql.user SET Password=PASSWORD('your_new_pass') WHERE User='root';
DELETE FROM mysql.user WHERE User='root' AND Host NOT IN ('localhost', '127.0.0.1', '::1');
DELETE FROM mysql.user WHERE User='';
DELETE FROM mysql.db WHERE Db='test' OR Db='test\\_%'
DROP DATABASE test;
FLUSH PRIVILEGES;

现在退出并返回mysql: mysql -u root -p

下面是MySQL 5.7的更新

bash --version
GNU bash, version 4.4.12(1)-release (x86_64-apple-darwin17.0.0)
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

#========================================
brew --version
Homebrew 1.7.6
Homebrew/homebrew-core (git revision eeb08; last commit 2018-09-27)
Homebrew/homebrew-cask (git revision c9f62; last commit 2018-09-27)

#========================================
mysql --version
mysql  Ver 14.14 Distrib 5.7.23, for osx10.13 (x86_64) using  EditLine wrapper

#========================================
system_profiler SPSoftwareDataType
Software:

    System Software Overview:

      System Version: macOS 10.13.3 (17D47)
      Kernel Version: Darwin 17.4.0
      Boot Volume: Macintosh HD
      Boot Mode: Normal
      Computer Name: EdisonMacHomeBj
      User Name: Edison (edison)
      Secure Virtual Memory: Enabled
      System Integrity Protection: Disabled
      Time since boot: 6 days 23:13

brew remove mysql@5.7
brew cleanup
mv /usr/local/var/mysql /usr/local/var/mysql.bak
brew install mysql@5.7
rm -rf /usr/local/var/mysql

#========================================
mysqld --initialize
2018-09-28T04:54:06.526061Z 0 [Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).
2018-09-28T04:54:06.542625Z 0 [Warning] Setting lower_case_table_names=2 because file system for /usr/local/var/mysql/ is case insensitive
2018-09-28T04:54:07.096637Z 0 [Warning] InnoDB: New log files created, LSN=45790
2018-09-28T04:54:07.132950Z 0 [Warning] InnoDB: Creating foreign key constraint system tables.
2018-09-28T04:54:07.196824Z 0 [Warning] No existing UUID has been found, so we assume that this is the first time that this server has been started. Generating a new UUID: 87cf2f10-c2da-11e8-ac2d-ba163df10130.
2018-09-28T04:54:07.224871Z 0 [Warning] Gtid table is not ready to be used. Table 'mysql.gtid_executed' cannot be opened.
2018-09-28T04:54:07.366688Z 0 [Warning] CA certificate ca.pem is self signed.
2018-09-28T04:54:07.457954Z 1 [Note] A temporary password is generated for root@localhost: kq3K=JR8;GqZ

#========================================
mysql_secure_installation -uroot -p"kq3K=JR8;GqZ"
mysql_secure_installation: [Warning] Using a password on the command line interface can be insecure.

Securing the MySQL server deployment.


The existing password for the user account root has expired. Please set a new password.

New password:

Re-enter new password:

VALIDATE PASSWORD PLUGIN can be used to test passwords
and improve security. It checks the strength of password
and allows the users to set only those passwords which are
secure enough. Would you like to setup VALIDATE PASSWORD plugin?

Press y|Y for Yes, any other key for No: n
Using existing password for root.
Change the password for root ? ((Press y|Y for Yes, any other key for No) : y

New password:

Re-enter new password:
By default, a MySQL installation has an anonymous user,
allowing anyone to log into MySQL without having to have
a user account created for them. This is intended only for
testing, and to make the installation go a bit smoother.
You should remove them before moving into a production
environment.

Remove anonymous users? (Press y|Y for Yes, any other key for No) : y
Success.


Normally, root should only be allowed to connect from
'localhost'. This ensures that someone cannot guess at
the root password from the network.

Disallow root login remotely? (Press y|Y for Yes, any other key for No) : n

 ... skipping.
By default, MySQL comes with a database named 'test' that
anyone can access. This is also intended only for testing,
and should be removed before moving into a production
environment.


Remove test database and access to it? (Press y|Y for Yes, any other key for No) : n

 ... skipping.
Reloading the privilege tables will ensure that all changes
made so far will take effect immediately.

Reload privilege tables now? (Press y|Y for Yes, any other key for No) : y
Success.

All done!

只是在之前的回答中添加一些东西-当从MySql 5.6升级到MySql 8.0时，我遵循了这里提供的步骤来进行干净的卸载，但我得到了以下错误

2019-11-05T07:57:31.359304Z 0 [ERROR] [MY-000077] [Server] /usr/local/Cellar/mysql/8.0.18/bin/mysqld: Error while setting value 'ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION' to 'sql_mode'.
2019-11-05T07:57:31.359330Z 0 [ERROR] [MY-013236] [Server] The designated data directory /usr/local/var/mysql is unusable. You can remove all files that the server added to it.
2019-11-05T07:57:31.359413Z 0 [ERROR] [MY-010119] [Server] Aborting
2019-11-05T07:57:31.359514Z 0 [Note] [MY-010120] [Server] Binlog end

我花了些时间才想明白。在这里找到了一个线索: https://discourse.brew.sh/t/clean-removal-of-mysql/2251

因此，我的问题的关键是在卸载后删除/usr/local/etc/my.cnf文件。 在完成最后一步之后，MySql终于开始工作了。

我认为如果已经安装了旧版本的mysql，就会出现这种情况。我也遇到了同样的问题，上面的解决方案对我都不起作用。我是这样安排的:

使用brew的remove & cleanup命令，卸载launchctl脚本，然后删除mysql目录在/usr/local/var，删除我现有的/etc/my.cnf(留给你，如果它适用)和launchctl plist

更新plist的字符串。还要注意，您的备用安全脚本目录将基于您正在安装的MySQL版本。

循序渐进:

brew remove mysql

brew cleanup

launchctl unload -w ~/Library/LaunchAgents/homebrew.mxcl.mysql.plist

rm ~/Library/LaunchAgents/homebrew.mxcl.mysql.plist

sudo rm -rf /usr/local/var/mysql

然后我从头开始:

installed mysql with brew install mysql ran the commands brew suggested: (see note: below) unset TMPDIR mysql_install_db --verbose --user=`whoami` --basedir="$(brew --prefix mysql)" --datadir=/usr/local/var/mysql --tmpdir=/tmp Start mysql with mysql.server start command, to be able to log on it Used the alternate security script: /usr/local/Cellar/mysql/5.5.10/bin/mysql_secure_installation Followed the launchctl section from the brew package script output such as, #start launchctl load -w ~/Library/LaunchAgents/homebrew.mxcl.mysql.plist #stop launchctl unload -w ~/Library/LaunchAgents/homebrew.mxcl.mysql.plist

注意:啤酒清理的——force位也会清理过时的酒桶，这是一个新的自制啤酒功能。

注意第二点:一个评论者说第二步不是必需的。我不想测试它，所以是YMMV!

如果brew安装了MySQL 5.7，这个过程与以前的版本有点不同。 重置root用户密码的操作步骤如下:

sudo rm -rf /usr/local/var/mysql
mysqld --initialize

临时密码将被打印到控制台，它只能用于更新根密码:

mysql.server start
echo "ALTER USER 'root'@'localhost' IDENTIFIED BY 'my-new-password';" | mysql -uroot --password=TEMPORARY_PASSWORD

Mysql的“Base-Path”存储在/etc/my.cnf中，当你进行升级时，它不会更新。只需打开它并更改basedir值

例如，修改如下:

[mysqld]
basedir=/Users/3st/homebrew/Cellar/mysql/5.6.13

指向新版本:

[mysqld]
basedir=/Users/3st/homebrew/Cellar/mysql/5.6.19

重新启动mysql:

mysql.server start