GROUP CONCAT and GROUP BY using LEFT JOIN If no record exist Must return something

I have 2 tables with below structures. question table id, audio, s_id 1, ad1, 500 2, ad2, 500 3, ad3, 500 answers table where q_id is FK of question table. id, s_id, q_id, number, answer 1, 500, 1, 088888801, 1 1, 500, 3, 088888801, 3 1, 500, 1, 090078601, 2 1, 500, 2, 090078601, 4 1, 500, 3, 090078601, 2 Lets take this example for better understanding of above structure. I have generated phone calls on 2 numbers (090078601, 088888801), They both have picked up the call. On call, There will be total of 3 questions that will play. As you can see the answer table, Number 088888801 has answered question 1 and 3, he has skip question number 2, if user skip a question, there will be no entry in answer table. But number 090078601 has answered all 3 questions. I want to fetch all record but if a user has skipped the answer, I want to show that this number has skipped that question, Here is my query SELECT r.number, GROUP_CONCAT(COALESCE(r.answer, 'skip')) as answer FROM questions LEFT JOIN answers ON r.q_id= q.id WHERE q.s_id= 500 group by r.number order by q.id ASC This query returns me below | number | answer | | -------- | ----- | | 088888801 | 1,3 | | NULL | NULL | | 090078601 | 2,4,2 | What I actually want is | number | answer | | -------- | ----- | | 088888801 | 1,'skip', 3 | | 090078601 | 2,4,2 | NOTE: I want an optimized query because There can be around 24 questions and more than 200000 numbers, If each number answers 24 questions there will be 4.8M records in answer table. This is just for one S_ID, There will be more records for other s_id, But I want query only on s_id because s_id is unique for each survey that runs.