First, you’ve to start MAMP or MAMP PRO, and you open your terminal and type:

/Applications/MAMP/Library/bin/mysql -uroot -p

Enter the password, by default the password, is root:

Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 254
Server version: 5.6.35 MySQL Community Server (GPL)
Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

If you want to use the command mysql by default without typing the entire command line you can edit one of these dot files ~./bashrc if you’re using bash or ~/.zshrc in case of using zsh and add this alias:

alias mysql=/Applications/MAMP/Library/bin/mysql

After editing the file using Vim or Nano save the dot file, and run the source to validate the modification:

If you use zsh:

source ~/.zshrc

If you use bash:

source ~/.bashrc

Now you can run the command mysql from the terminal and enter the username and the password:

mysql -uroot -proot