If you install mysql via Homebrew ($ brew install mysql) and start it via Homebrew services ($ brew services start mysql), chances are that you’ll see an error when trying run mysql -u root on the command line.
If that error is command not found: mysql, the issue is likely that you need to add the Homebrew mysql directory to your PATH.
On my machine, Homebrew installed mysql here:
/usr/local/Cellar/mysql@5.7/5.7.25/
To add this to your PATH, add this to your .bash_profile (or .zhsrc if you use ZSH):
export PATH=/usr/local/Cellar/mysql@5.7/5.7.25/bin:$PATH
Then, reload your shell and type mysql -u root to confirm all is working.
You can quit the mysql CLI via mysql> \q
