SQL view query not working - mysql

Question

These are my tables:
portal_users(id, first_name, last_name, email, ...,)
courses(id, name, location, capacity, ...,)
feedback_questions(id, question, ...,)
feedback_answers(id, answers, IDquestion, IDuser, IDcourse)
I want to do a view with this:
course | first_name | last_name | IDuser | IDcourse | question_1 | answer_1 | question_2 | answer_2
So far
CREATE VIEW feedback_answers_vw
as
SELECT
fa.id,
fa.answer,
fq.question,
pu.first_name,
pu.last_name,
fa.IDuser,
fa.IDcourse
FROM feedback_answers fa
INNER JOIN feedback_questions fq
ON fa.IDquestion = fq.id
INNER JOIN portal_users pu
ON fa.IDuser = pu.id
INNER JOIN courses cu
on fa.IDcourse = cu.id
GROUP BY
fa.IDcourse, fa.IDuser
This just display one question and its answers, but not all the questions that belong to the same course and user.
I could hardcode this with something like this in the SELECT statement
SELECT
fa.id,
(select question
from feedback_questions
where id = 1) as question_1,
(select question
from feedback_questions
where id = 2) as question_2,
(select question
from feedback_questions
where id = 3) as question_3,
pu.first_name,
pu.last_name,
fa.IDuser,
fa.IDcourse
But I want to do it in the right way, so I won't be changing the code everytime that a question is added.
Edit:
This is data example of my tables:
**Portal users:**
1, tom, hanks, tom_hanks#example.com, ...,
2, steven, spielberg, steven#example.com, ...,
**Courses:**
1, quality, california, 30
2, information technologies, texas, 24
**Questions:**
1, How did you find the course?, ...,
2, Do you want purchase order?, ...,
**Answers:**
1, Internet, 1, 1, 1
2, yes, 2, 1, 1
3, TV, 1, 2, 1,
4, no, 2, 2, 1,
5, Internet, 1, 1, 2
6, yes, 1, 1, 2
This are data example I want to display in the view:
course|first_name|last_name|IDuser|IDcourse|Question_1|Answer_1|Question_2|Answer_2
----------------------------------------------------------------------------------
quality | tom | hanks | 1 | 1 | How did you find the course? | Internet | Do you want purchase order? | yes
quality | steven | spielberg | 2 | 1 | How did you find the course? | TV | Do you want purchase order? | no
Information technologies | tom | hanks | 1 | 2 How did you find the course? | Internet | Do you want purchase order? | yes

Answer 1

I dont know if you would actually need it as a view, and here is a sample query for it.
If you look at the first part of the where clause, it is based on a minimum of question ID = 1. I then join that to the course table and portal_user table to get those fields. Since the feedback answers is the first table, we can immediately get that answer, but also join to the questions table to get the question.
So now, how to simply expand this query for future questions? Notice the LEFT-JOINs right after that for the feedback answers AGAIN, but this time as an alias "fa2" (feedback_answers2), and the JOIN clause is based on the same user, same course, but only for question ID = 2. This then joins to the questions table on the fa2.questionID to the questions table (alias fq2). And then another exact same setup but for question 3. So here, I am doing nothing but using the same tables, but just different aliases. They all start with question 1, and if there is a question 2, get it... if a question 3, get it too, expand as needed, no group by, etc.
So now if you only cared about a single course you were interested, just add that to the outermost WHERE clause, nothing else needs to change. Get 10 questions, copy/paste the LEFT-JOIN components for the corresponding question IDs.
SELECT
c.name as course,
pu.first_name,
pu.last_name,
fa1.IDUser,
fa1.IDCourse,
fq1.question as question1,
fa1.answers as answer1,
fq2.question as question2,
fa2.answers as answer2,
fq3.question as question3,
fa3.answers as answer3
from
feedback_answers fa1
JOIN courses c
ON fa1.IDCourse = c.id
JOIN portal_users pu
ON fa1.IDUser = pu.id
JOIN feedback_questions fq1
ON fa1.IDquestion = fq1.id
LEFT JOIN feedback_answers fa2
ON fa1.IDUser = fa2.IDUser
AND fa1.IDCourse = fa2.IDCourse
AND fa2.id = 2
LEFT JOIN feedback_questions fq2
ON fa2.IDquestion = fq2.id
LEFT JOIN feedback_answers fa3
ON fa1.IDUser = fa3.IDUser
AND fa1.IDCourse = fa3.IDCourse
AND fa2.id = 3
LEFT JOIN feedback_questions fq3
ON fa3.IDquestion = fq3.id
where
fa1.id = 1

Answer 2

After some sample data was posted, it is clear a pivot table (in access/excel known as a 'crosstab') is needed here. We could hard code a select with some case statements to cover a fixed number of questions, but that solution would have to be revisited each time a question was added or removed. #drapp has such a solution below.
However, what we really want is the number of columns returned by the view to grow and shrink with the number of questions. Unfortunately mysql doesn't have any built in functions to help us build one. The best solution I found online is http://www.artfulsoftware.com/infotree/qrytip.php?id=523. The author proposes building the sql select statement dynamically then executing it. It is actually a pretty elegant solution.
Faced with this requirement, I wouldn't do this work in SQL. The problem I foresee is that as the numbers of questions rises, more and more columns will be returned; your view would quickly slow down to a crawl, and eventually stop working altogether. I would try to transform the data outside of sql, perhaps in an etl tool, application server, or BI tool.
Or, if I had the ability to (and I never do), I would switch database engines. Here are three solutions from other engines that provide tools for creating pivot tables:
Oracle: http://www.oracle.com/technetwork/articles/sql/11g-pivot-097235.html
Postgres: http://www.postgresql.org/docs/9.1/static/tablefunc.html
SQL Server: http://blogs.msdn.com/b/spike/archive/2009/03/03/pivot-tables-in-sql-server-a-simple-sample.aspx

Answer 3

We just talked about views in my DB Design class and my Professor said that you can't use a join in a VIEW statement. I can't recall why not but your issues looks familiar.