SQL Tutorial
Interactive SQL tutorial (SQLzoo.net)
SQL Quick Reference
Database Normalization Basics
http://www.geekinterview.com/quiz/SQLQuiz.html
Oracle specific, includes questions about SQL Plus commands.
http://www.wiziq.com/tests/sql
Mostly Oracle specific, need to sign up to see your score.
http://www.sqlquiz.com/
Very good!
After taking the test you get a report with your time, your score, your answer for each question, if this is correct or not, and recommended reading for incorrect answers.
http://tools.people-centric.ro/index.php?option=com_otests&view=order
http://www.w3schools.com/sql/sql_quiz.asp
20 questions, no time limit, at the end of the Quiz, your total score will be displayed.