Question

Let’s suppose I have two tables:

Question (Id, Text)
Answer (Value, QuestionId, Guid)

Column Guid groups answers from the same person.

I need a query to produce results like this:

'Question 1' | 'Question 2'
4            | 3
1            | NULL
NULL         | 5
2            | 6
9            | NULL

Questions texts are transformed into column headers and answers values are in rows. Answers are grouped by Guid, so there are answers from one person in one row. If a person didn’t answer particular question, NULL is returned.

Number of questions can vary.

Data used to produce sample results:

 Question
 Id   | Text
 1    | Question 1
 2    | Question 2

 Answer
 Value | QuestionId | Guid
 4     | 1          | AAA
 3     | 2          | AAA
 1     | 1          | BBB
 5     | 2          | CCC
 2     | 1          | DDD
 6     | 2          | DDD
 9     | 1          | EEE

Can you please help me out with a query to produce the results?

Answer 1

Since you have an unknown number of questions then tou will need to use dynamic SQL to PIVOT this:

DECLARE @colsFinal AS NVARCHAR(MAX),
    @colsPivot AS NVARCHAR(MAX),
    @query  AS NVARCHAR(MAX)

select @colsFinal = STUFF((SELECT distinct ',' 
                      + QUOTENAME(Id) 
                           + ' as Question_'+ cast(Id as varchar(10))
                    from question
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

select @colsPivot = STUFF((SELECT distinct ',' 
                      + QUOTENAME(Id) 
                    from question
            FOR XML PATH(''), TYPE
            ).value('.', 'NVARCHAR(MAX)') 
        ,1,1,'')

set @query = 'SELECT ' + @colsFinal + ' from 
             (
                select questionid, value, guid
                from question q
                left join value v
                  on q.id = v.questionid
            ) x
            pivot 
            (
                min(value)
                for questionid in (' + @colsPivot + ')
            ) p '

execute(@query)

See SQL Fiddle with Demo

If you had a known number of columns, then you could hard-code the values for the PIVOT (See SQL Fiddle With Demo):

select [1] as Question1, [2] as Question2
from 
(
  select questionid, value, guid
  from question q
  left join value v
    on q.id = v.questionid
) x
pivot
(
  max(value)
  for questionid in ([1], [2])
) p

Or you can use an aggregate function with a CASE (See SQL Fiddle With Demo):

select max(case when q.id = 1 then v.value end) Question1,
  max(case when q.id = 2 then v.value end) Question2
from question q
left join value v
  on q.id = v.questionid
group by guid