Perhaps generate individual keys using some secure random process. Then you can check your logs now and again and if a few particular keys are submitting obviously bogus data you can just remove them from your records and block them in future.

As has been mentioned elsewhere there are other risks by the fact that your allowing people to enter data into your system using an actual language (SQL) as opposed to a few POST vars (assuming your webservice sanitizes input properly).

There are just many more possible attack vectors with mysql , one of which would be sending massive crossjoins or similar to the DB to crash it.