help with quiz project

hi everyone.. I'm trying to complete a quiz project in which users log in to the website, see the questions, answer them and finally see the results. so far I have a questions table which stores my questions containing these columns: QuestionId, Text, Choices, CorrectAnswer,ExamId, ... and an answers table which stores every users answer containing AnswerId, UserId, QuestionId, UserAnswer,ExamId... my final goal is to display the users score and ranking in the results page... for counting the correct, incorrect and null answers I JOIN the Answers Table on the questions table and compare the users answer to the correct one and use this data to calculate the users score percentage..  
now the problem is that I can't find a way to calculate all of the users scores at once and give a specific user its proper ranking among others.. should I design another table for doing this task or can I do it in the same page and with my available tables?
i'm using ...