Use a normal web page to post data into your database.
Create a web service that exposes the data you want, in response to a specific GET request.
Have your Arduino running as a web client and periodically poll the web service. If there is data for the Arduino then it can parse the web service response and act on the data.
Decide on your polling interval based on how fast you need the Arduino to act to new commands posted into your database.
Once the Arduino has carried out the command you can even have it post back, again via a web service, into your database to confirm receipt and action to close the communication loop.
All nice and simple stuff for any web developer. :-)