How would I perform math within a sql query to calculate percent difference?

Question

I would like to take surveys (2 or more choices), and place the results in a table. I just want to know how I would tally up the results in a sql query.

Even more, should i be doing the math within the query, or just use PHP to do the math?

Example

Questions Table

question_id (int)
question (text)
answer_1 (varchar)
answer_2 (varchar)
answer_3 (varchar)
etc...

Answers Table

answer_id (int)
question_id (int)
answer (int) The answer they chose. (1, 2, 3, etc.)

How would/should I tally up the results?

Edit: using MySQL, PHP. The difference between the answers (45% said blah, 50% said blah blah, 5% said bloch). Not for homework, Im EE, not CS

Answer 1

This will select popularity of each answer within the question:

SELECT  question,
        COUNT(*) /
        (
        SELECT  COUNT(*)
        FROM    answers ai
        WHERE   ai.question = a.question
        )
FROM    answers a
GROUP BY
        question, answer

This will not selects the answers if they had never been given.

Answer 2

select answer, count(*) from Answers where answer_id = 1 group by answer

or

select answer_id,answer, count(*) from Answers  group by answer_id, answer

I'd give you a query that counts the answers to each question, but sadly you don't have a way to relate the answers to the questions. Nor should you have answer_1, answer_2, etc in the questions table. You need to normalize tha table. If you don't understand how to do it, you need to learn before designing a database.

Answer 3

If the number of answers is not known beforehand, it would be simpler to split the questions table into 2 - one for the questions (question_id, question_text), and one for choices (question_id, choice_id, choice_text). The answers table could be made into (question_id, answer_id, choice_id). Then selecting it would be using something like the following (QID = the ID of the question you're selecting):

SELECT choice,
       (COUNT(*) / (SELECT COUNT(*)
                    FROM answers
                    WHERE answers.question_id = QID)) * 100 AS percentage
FROM choices
     INNER JOIN answers
       ON choices.choice_id = answers.choice_id
       AND choices.question_id AND choices.question_id
WHERE choices.question_id = QID
GROUP BY choice_id;

All this does is count the total number of answers in the inner query, then for each choice, divide the number of answers with that choice over the total.