I tried out using the Yun to read and write to an Amazon RDS Mysql database by following the Temboo how-to at https://temboo.com/arduino/mysql
Be warned: you have to be very careful that you don't go over the free limits. I got a bill for $485 for November! The free tier structure is hard to figure out, so forewarned is forearmed.
BTW, they refunded my money when I called to complain, since I had queried the database about 30 times total.