This is an empty package that depends on the current "best" version of mysql-server (currently mysql-server-5.7), as determined by the mysql maintainers. install this package if in doubt about which mysql version you need. that will install the version recommended by the package maintainers.
mysql is a fast, stable and true multi-user, multi-threaded sql database server. sql (structured query language) is the most popular database query language in the world. the main goals of mysql are speed, robustness and ease of use.