The MySQL plugin enables the Pawn runtime to communicate with a MySQL server in an asynchronous way, in order to not block execution of the rest of the gamemode. The plugin has currently been build against MySQL 5.5.
See BUILD.md for instructions on how to build this plugin on Windows. On Linux, just running make in the src directory should work.
See a_mysql.pwn for the native functions exposed by this function, and a generic explanation on how to use this plugin.