If you install LAMP/WAMP on your PC, this includes the MySQL database. There is an Arduino library that can connect directly to a MySQL database and store data into the database (the "M" in LAMP stands for MySQL). Alternatively, the Arduino can send an http GET or POST request to the PC. A short PHP script on the PC can receive these requests and perform the database update (the "P" in LAMP stands for PHP, which is a scripting language).