MySQL: How to Add an Existing User to an Existing Database
GRANT USAGE ON `<DATABASE_NAME>`.* TO '<USERNAME>'@'%';
GRANT ALL PRIVILEGES ON `<DATABASE_NAME>`.* TO '<USERNAME>'@'%' WITH GRANT OPTION;
GRANT USAGE ON `<DATABASE_NAME>`.* TO '<USERNAME>'@'%';
GRANT ALL PRIVILEGES ON `<DATABASE_NAME>`.* TO '<USERNAME>'@'%' WITH GRANT OPTION;