MySQL Query select all where only one of two options are found

Question

I have two tables:

Members ( member_id , member_info )

Question_Answers ( Question_id , Answer )

If there are two questions, I need to find out which member only answered one of the questions, and not both.

I have tried to use GROUP BY , but have not gotten the results I am looking for.

Closest I've gotten was with this query:

SELECT DISTINCT(qa.question_id), mem.member_id
FROM question_answers qa
LEFT JOIN members mem ON mem.member_id = qa.member_id

This gets me the results for questions answered by user, but it includes results where the members answered both questions.

I am looking for advice on how to SELECT the member that only answered one of the questions.

EDIT: I have gotten closer, but would like to make it better.

    SELECT IF( COUNT( DISTINCT( qa.question_id ) ) = 2, 'TWO', 'ONE' ), mem.member_id, MAX( qa.question_id )
FROM question_answers qa
JOIN members mem ON qa.member_id = mem.member_id
WHERE mem.type = 9
GROUP BY mem.member_id  
ORDER BY `mem`.`member_id` ASC

This is a hacky way because the question ID's are currently on 0 and 1, but if anyone has a recommendation on how to return the question_id from the SELECT IF( then that would help out a lot.

Answer 1

Personally, I would have a third table to hold the data of who has answered which questions.

In my example I have called this table Answered .

Here is how I would solve this, maybe you can glean some ideas off this example if you don't want to do it this way: [DEMO HERE] .

SELECT m1.*, qa.*
FROM Members m1
INNER JOIN Answered a ON m1.member_id = a.member_id
INNER JOIN Question_Answers qa ON a.question_id = qa.question_id
WHERE m1.member_id IN (
    SELECT m2.member_id
    FROM Members m2
    INNER JOIN Answered a ON m2.member_id = a.member_id
    INNER JOIN Question_Answers qa ON a.question_id = qa.question_id
    GROUP BY m2.member_id
    HAVING COUNT(m2.member_id) = 1);