Question

Hi I’m currently developing a sort of quiz application. In “Discover” section I get popular and latest questions. But when I get them I should also check if question is already answered by current user or not.

Here are my tables:
questions: (media_id is primary key, player_id is creator of the question)

media_id - player_id - answer0 - answer1 - ...

answers: (media_id is primary key, player_id is the person who answers the question)

media_id - player_id - result - ...

This is how I get popular questions:

select * from questions where order by popularity_count

EDIT:
Let me show what I want to achieve by examples:

questions rows:

media_id - player_id - answer0 - answer1
0123456    abc123      bla       bla
6543210    hjk789      lor       ips

answers rows:

media_id - player_id - result
6543210    abc123      1

So when user “abc123” gets the popular questions results should be:

media_id - player_id - answer0 - answer1 - is_answered
0123456    abc123      bla       bla       0
6543210    hjk789      lor       ips       1

The problem is I need a temporary column in the result of query that indicates if question is replied by the current user or not. How can I achieve this?

I can find if question is already answered or not for popularity section by using two queries. First I get the popular questions then I simply check if question is answered or not for every questions which seems so ineffective way. How can I achieve this with only one query?

Thank you!

Answer 1

It doesn’t seem you are approaching this problem correctly as you have a non-normalized table structure.

You should have a separate table that contains the answers by row.

questions (id, media_id, player_id, question)

answers (id, media_id, player_id, question_id, answer)

This way you can LEFT JOIN answers to questions and easily see which questions are not answered like so:

SELECT q.question, a.answer
FROM question q
LEFT JOIN answers a ON a.question_id = q.id
  AND a.media_id = q.media_id
  AND a.player_id = q.player_id

See it in action

To add a column to your current query, you can append it to your SELECT and use a CASE statement to check whether it’s been answered.

SELECT ..., 
    CASE
        WHEN answer0 IS NOT NULL
            THEN 'Yes'
        ELSE 'No'
    END AS 'Answered'
FROM ...

Update

Based on your update, it seems as though you are only JOINing on player_id. I believe you intend to JOIN on both player_id and media_id. See the difference between the data you provided and what I think you intend it to be.