Question

sorry about the title, I’m not sure how to even describe this, which makes it even harder to search for a solution.

I have a table which has many answers:

CREATE TABLE `answers` (
  `a_id` int(11) NOT NULL auto_increment,
  `p_id` int(11) NOT NULL default '0',
  `q_id` int(11) NOT NULL default '0',
  `user_id` int(11) NOT NULL default '0',
  `correct` int(1) NOT NULL default '0',
  `timestamp` timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP,
  PRIMARY KEY  (`a_id`)
) ;

I need to select a q_id from a specific user_id and p_id where correct = 0, but only where a more recent row from the same user_id and p_id’s correct field is not 1.

I can group by id, but am unsure how to eliminate groups where the top correct != 0

Thanks in advance. I’ve found many helpful answers here and am looking forward to contributing where I can.

EDIT: Currently the query as stands, but takes 6 secs to execute:

From both your answers I have query that works, but takes 6 secs to execute!

SELECT a.q_id FROM answers a    
JOIN (SELECT b.q_id, MAX(b.a_id) as a_id, b.correct
        FROM answers b  
       WHERE b.correct = 1 
         AND b.user_id = 1 
         AND b.p_id = 22
    GROUP BY b.q_id) c ON a.q_id = c.q_id 
                      AND a.a_id > c.a_id
WHERE a.correct = 0 
  AND a.user_id = 1 
  AND a.p_id = 22  
LIMIT 1

With no JOIN they take .26 secs. and .45 secs to execute How can I make it more efficient?

Is there another way to select the most recent correct = 0 row where a prev correct = 1 row does not exist?

Thanks for all your help!

Answer 1

This query seems to be working best. It’s my edited answer, updated with ordering the a.a_id and limiting the return to one.

  SELECT a.q_id 
      FROM user_q_answers a    
      JOIN (SELECT b.user_id,
                   b.popling_id, 
             b.q_id,
                   MAX(b.a_id) as a_id
              FROM user_q_answers b  
             WHERE b.correct = 1 
          GROUP BY b.q_id, b.user_id) c ON a.q_id = c.q_id
                                       AND a.user_id = c.user_id
                                       AND a.a_id > c.a_id
     WHERE a.correct = 0 
       AND a.user_id = 101
       AND a.popling_id = 170  
    ORDER BY a.a_id DESC
    LIMIT 1

Thanks very much to OMG Ponies and Nirmal for helping me through this. Sorry if answering my own question is bad form here, but I comprised my query based on what I learned from you both.

Thanks again!!!