Create a MySQL database with user and privileges

sudo mysql

Create the database, user, and privileges:

CREATE DATABASE app_db CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
CREATE USER 'app_user'@'localhost' IDENTIFIED BY 'strong-password';
GRANT ALL PRIVILEGES ON app_db.* TO 'app_user'@'localhost';
FLUSH PRIVILEGES;

Then test the new credentials:

mysql -u app_user -p app_db