I don't have any links to free practice tests, but I do have a link to a site for getting some practice in writing sql queries.
Here is a site for practicing sql queries though.
I'd agree with supa on his recommendations on practice exams. What you get for free on the net will not come close to equaling the difficulty of the questions on the real exams. Transcender and Self Test will give you a very good approximation of real tests. They are spendy, but well worth the money.