Test Your SQL Performance Know-How in 3 Minutes - the author says 60% of those who take this little quiz fail! You can select a database of your choice and the quiz will grill you on knowing the basics of indexes and when a query can make good use of particular indexes.
I chose Postgres as my database of choice and passed with a 4 out of 5. I attribute my mistake to a misreading but whatever, I have to accept that I messed one up. :)