选择NOT EXISTS和CASE的随机行

Question

I have three tables in my database for questions, answers and viewed:

Questions:

---------------------------------
| ID | category | title         |
---------------------------------
| 1  | 1        | What is this? |
---------------------------------
| 2  | 1        | Who is this?  |
---------------------------------

And so on: category is a category id from a different table, but thats not important.

Then I have answers:

-------------------------------------------
| ID | question | answer | user | date    |
-------------------------------------------
| 1  | 1        | Blue   | 25   | 1.1.15  |
-------------------------------------------
| 2  | 1        | Red    | 18   | 2.2.15  |
-------------------------------------------
| 3  | 2        | Brad   | 25   | 1.1.15  |
-------------------------------------------

Where question is the id from questions table and user is user_id

Then I have viewed:

-----------------------------------
| ID | category | question | user |
-----------------------------------
| 1  | 1        | 1        | 25   |
-----------------------------------
| 2  | 1        | 1        | 18   |
-----------------------------------
| 3  | 1        | 2        | 25   |
-----------------------------------

This will keep track of which questions users have seen, so they don't see the same question twice. And the point is that a user can give their answer that another user have given, so that won't be stored in answers table.

I have tried with this MYSQL-code:

SELECT Q.ID, Q.category, Q.title, A.ID, A.answer
FROM questions Q, answers A
WHERE
NOT EXISTS (
    SELECT *
    FROM category_answered S
    WHERE Q.ID = S.question AND S.user = '$user' AND S.category = '$category'
)
AND
CASE
WHEN Q.category = '$category' THEN Q.ID = A.question
ELSE Q.category = '$category'
END
ORDER BY RAND() LIMIT 1

This list everything except the question that matches. So ORDER BY RAND() LIMIT 1 is not working.

Any help will be gladly appreciated