How to validate if product key is in mysql

I am working on a game which I am going to sell, and I have system on my website that will give people who buy the game a code and the code goes in a mySQL DB, and I want to know how to make it so when people launch the game (FOR THE FIRST TIME ONLY) how to make it so they need to input their product key and then it verifies if its real or not and they don't need to input it again unless whatever storing the info needed is not there anymore. I also want to know how to verify if the code is real every time they launch the game and if it isn't to exit the game with a popup saying "Invalid licence key" or something like that. Any help would be great, Thank you!