MySQL – Creating Column Names from Other Table Rows

I have found a clever way to do this query without a self join.

I ran these commands in MySQL 5.5.8 for Windows and got the following results:

use test
DROP TABLE IF EXISTS answers;
CREATE TABLE answers (user_id VARCHAR(10),question_id INT,answer_value VARCHAR(20));
INSERT INTO answers VALUES
('Sally',1,'Pouch'),
('Sally',2,'Peach'),
('John',1,'Pooch'),
('John',2,'Duke');
INSERT INTO answers VALUES
('Sally',1,'Pooch'),
('Sally',2,'Peach'),
('John',1,'Pooch'),
('John',2,'Duck');

SELECT user_id,question_id,GROUP_CONCAT(DISTINCT answer_value) given_answers
FROM answers GROUP BY user_id,question_id;

+---------+-------------+---------------+
| user_id | question_id | given_answers |
+---------+-------------+---------------+
| John    |           1 | Pooch         |
| John    |           2 | Duke,Duck     |
| Sally   |           1 | Pouch,Pooch   |
| Sally   |           2 | Peach         |
+---------+-------------+---------------+

This display reveals that John gave two different answers to question 2 and Sally gave two different answers to question 1.

To catch which questions were answered differently by all users, just place the above query in a subquery and check for a comma in the list of given answers to get the count of distinct answers as follows:

SELECT user_id,question_id,given_answers,
(LENGTH(given_answers) - LENGTH(REPLACE(given_answers,',','')))+1 multianswer_count
FROM (SELECT user_id,question_id,GROUP_CONCAT(DISTINCT answer_value) given_answers
FROM answers GROUP BY user_id,question_id) A;

I got this:

+---------+-------------+---------------+-------------------+
| user_id | question_id | given_answers | multianswer_count |
+---------+-------------+---------------+-------------------+
| John    |           1 | Pooch         |                 1 |
| John    |           2 | Duke,Duck     |                 2 |
| Sally   |           1 | Pouch,Pooch   |                 2 |
| Sally   |           2 | Peach         |                 1 |
+---------+-------------+---------------+-------------------+

Now just filter out rows where multianswer_count = 1 using another subquery:

SELECT * FROM (SELECT user_id,question_id,given_answers,
(LENGTH(given_answers) - LENGTH(REPLACE(given_answers,',','')))+1 multianswer_count
FROM (SELECT user_id,question_id,GROUP_CONCAT(DISTINCT answer_value) given_answers
FROM answers GROUP BY user_id,question_id) A) AA WHERE multianswer_count > 1;

This is what I got:

+---------+-------------+---------------+-------------------+
| user_id | question_id | given_answers | multianswer_count |
+---------+-------------+---------------+-------------------+
| John    |           2 | Duke,Duck     |                 2 |
| Sally   |           1 | Pouch,Pooch   |                 2 |
+---------+-------------+---------------+-------------------+

Essentially, I performed three table scans: 1 on the main table, 2 on the small subqueries. NO JOINS !!!

Give it a Try !!!

Considering that the question is labeled database-design I will offer a solution without that CSV field. Even if you decide to keep it, this may offer some ideas.

• Question contains all possible questions.
• Answer has all possible answers (to any question)
• QuestionAnswer contains all QA combinations offered, note that there must exist a default answer for "I do not know".
• UserQA contains questions user answered.
• Note that rule "There can be one and only one correct answer for each question" (IsCorrect = 'Y'), has to be implemented on the application level or as a stored procedure/trigger.

To get questions that a user with specific_UserId has not yet answered

select  qq.QuestionID
from Question as qq
where not exists (select 1 
                    from UserQA as xx 
                   where xx.QuestionID = qq.QuestionID
                     and xx.UserID     = specific_UserId );

Or, better if there may be some new questions which do not have defined answers yet

select distinct qq.QuestionID
from QuestionAnswer as qq
where not exists (select 1 
                    from UserQA as xx 
                   where xx.QuestionID = qq.QuestionID
                     and xx.UserID     = specific_UserId );