Ranking Users if Multiple Database search

Hey guys, im working on a dashboard for administrators and in this dashboard, i want to display the name of the Seller ( user) who sold the most ever ( not in a especific period). Im struggling with that, i have a database User, in this database there’s an cupom/code that is vinculated to another database called Salles where is registrated each individual salle. I nedd to sum every value of every salle of every user and compare which is the user who has the biggest salle. Is there anybody who can solve my problem?