Making a scheduled function to generate specified number of random numbers from with in a table MYSQL

I'm working on my website where people can do give-a-ways of game items. Now I want to create a scheduled task in mySQL database so when the time of give-a-way is over, it will randomly select a specified number of winners from a join table "participants" in which the id of users (as Uid) and the give-a-way(as gid) in which they have participated is stored. I have tried reading different articles but I don't have a clue how to do that since I haven't studied databases course in my degree.

Any help will be of great use. Thank you.