如何过滤出用户回答的问题？

Question

I am building a small quiz/test engine with AJAX, PHP, and MySQL. All of the data (questions, answers, etc.) is stored in a database.

The issue that I am having is that I can't find out how to filter out the questions that have been answered by the user that is logged in, therefore resulting in questions being repeated and a never ending quiz/test.

When you click the "next question" button, an AJAX request is sent to submit_answer.php which submits the answer (adds row to user_answers table) and it also sends a request to get_question.php which then returns JSON of the question information (question, answers, etc.) but I can't seem to get it to not select questions that have been answered (questions in the user_answers table). Here is what I've got right now:

$question = mysql_query("SELECT *, q.id qid, q.question question_text 
                         FROM questions q, user_answers ua
                         WHERE q.id != ua.question_id 
                           AND ua.test_id = $test 
                           AND ua.user_id = $_SESSION[userid] 
                         ORDER BY rand() 
                         LIMIT 1");

$q = mysql_fetch_assoc($question);

This query is supposed to select the questions that have the test_id of the current test, has the user_id as the current user, and not in the user_answers table. But apparently, I'm just not doing it right.

You can see the entire get_question.php file here: http://pastebin.com/Td6mqp49

Edit:

Here are my table structures as requested by @MadaraUchiha:

questions

user_answers

Answer 1

SELECT *, q.id qid, q.question question_text 
FROM questions q
WHERE NOT EXISTS (
    SELECT TRUE
    FROM user_answers ua
    WHERE ua.question_id = q.id
    AND ua.test_id = $test 
    AND ua.user_id = $_SESSION[userid]
)
ORDER BY rand() 
LIMIT 1

The reason why your query wasn't working is that q.id != ua.question_id will match as long as there is at least one answer that doesn't belong to the question - any answer.

Answer 2

You need a left outer join between the questions to the user_answers tables.

The final query looks like:

SELECT *, q.id qid, q.question question_text 
FROM questions q left outer join
     user_answers ua
     on q.id = ua.question_id and
        ua.test_id = $test and
        ua.user_id = $_SESSION[userid] 
WHERE ua.user_id is null
ORDER BY rand() 
LIMIT 1

Answer 3

SELECT * FROM
questions q
WHERE 
q.id NOT IN (
    SELECT question_id as id
    FROM user_answers
    WHERE ua.test_id = $test AND ua.user_id = $_SESSION[userid]
)

On a side note, make sure you properly sanitize the data before passing to the SQL.