** Create a new user and provide permissions to one database

CREATE USER 'new_user'@'localhost' IDENTIFIED BY 'new_password';
GRANT ALL ON my_db.* TO 'new_user'@'localhost';
FLUSH PRIVILEGES;

<< or >>

GRANT ALL PRIVILEGES ON my_db.* To 'new_user'@'localhost' IDENTIFIED BY 'password';
FLUSH PRIVILEGES;


using % instead of localhost would give user the privilege to access the db from anywhere.