SQL - MAX function returning incorrect data

Question

I am trying to get with each question that having max seq_no record, but it is returning wrong data.

Here is query:

SELECT id, attempt, seq_number, question
FROM question_states
WHERE attempt = 374
AND seq_number IN ( SELECT MAX(seq_number) FROM question_states WHERE attempt = 374 GROUP BY question)
GROUP BY question, id

I have one table name as questions, structure is given below:-

id--seq_number--attempt--question
1 --0     --374  -- 130
2 --0     --374  -- 130
3 --1     --374  -- 130
4 --2     --374  -- 130
5 --1     --374  -- 131

Why this query returns 2 records against question 130 ? and if I remove the GROUP BY id then returns wrong data.

Answer 1

Try this:

SELECT id, attempt, seq_number, question
FROM question_states qs 
INNER JOIN (SELECT question, MAX(seq_number) seq_number 
            FROM question_states WHERE attempt = 374 GROUP BY question
           ) AS A ON qs.question = A.question AND qs.seq_number = A.seq_number 
WHERE attempt = 374

OR

SELECT id, attempt, seq_number, question
FROM (SELECT id, attempt, seq_number, question
      FROM question_states WHERE attempt = 374
      ORDER BY question, seq_number DESC
     ) AS A 
GROUP BY question

Answer 2

In the subquery, since you used group by, it will return two values '2' and '1'. and in the main query since you used 'IN' statement, it will return all the data with field seq_number and attempt = 374

try

select max(seq_number), id, attempt, question from question_states group by seq_number ORDER by seq_number DESC limit 1