Join tables by column names, convert string to column name - sql-server

Question

I have a table which store 1 row per 1 survey.
Each survey got about 70 questions, each column present 1 question
SurveyID Q1, Q2 Q3 .....
1 Yes Good Bad ......
I want to pivot this so it reads
SurveyID Question Answer
1 Q1 Yes
1 Q2 Good
1 Q3 Bad
... ... .....
I use {cross apply} to acheive this
SELECT t.[SurveyID]
, x.question
, x.Answer
FROM tbl t
CROSS APPLY
(
select 1 as QuestionNumber, 'Q1' as Question , t.Q1 As Answer union all
select 2 as QuestionNumber, 'Q2' as Question , t.Q2 As Answer union all
select 3 as QuestionNumber, 'Q3' as Question , t.Q3 As Answer) x
This works but I dont want to do this 70 times so I have this select statement
select ORDINAL_POSITION
, COLUMN_NAME from INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = mytable
This gives me the list of column and position of column in the table.
So I hope I can somehow join 2nd statement with the 1st statement where by column name. However I am comparing content within a column and a column header here. Is it doable? Is there other way of achieving this?
Hope you can guide me please?
Thank you

Answer 1

Instead of Cross Apply you should use UNPIVOT for this query....
SQL Fiddle
MS SQL Server 2008 Schema Setup:
CREATE TABLE Test_Table(SurveyID INT, Q1 VARCHAR(10)
, Q2 VARCHAR(10), Q3 VARCHAR(10), Q4 VARCHAR(10))
INSERT INTO Test_Table VALUES
(1 , 'Yes', 'Good' , 'Bad', 'Bad')
,(2 , 'Bad', 'Bad' , 'Yes' , 'Good')
Query 1:
SELECT SurveyID
,Questions
,Answers
FROM Test_Table t
UNPIVOT ( Answers FOR Questions IN (Q1,Q2,Q3,Q4))up
Results:
| SurveyID | Questions | Answers |
|----------|-----------|---------|
| 1 | Q1 | Yes |
| 1 | Q2 | Good |
| 1 | Q3 | Bad |
| 1 | Q4 | Bad |
| 2 | Q1 | Bad |
| 2 | Q2 | Bad |
| 2 | Q3 | Yes |
| 2 | Q4 | Good |

Answer 2

If you need to perform this kind of operation to lots of similar tables that have differing numbers of columns, an UNPIVOT approach alone can be tiresome because you have to manually change the list of columns (Q1,Q2,Q3,etc) each time.
The CROSS APPLY based query in the question also suffers from similar drawbacks.
The solution to this, as you've guessed, involves using meta-information maintained by the server to tell you the list of columns you need to operate on. However, rather than requiring some kind of join as you suspect, what is needed is Dynamic SQL, that is, a SQL query that creates another SQL query on-the-fly.
This is done essentially by concatenating string (varchar) information in the SELECT part of the query, including values from columns which are available in your FROM (and join) clauses.
With Dynamic SQL (DSQL) approaches, you often use system metatables as your starting point. INFORMATION_SCHEMA exists in some SQL Server versions, but you're better off using the Object Catalog Views for this.
A prototype DSQL solution to generate the code for your CROSS APPLY approach would look something like this:
-- Create a variable to hold the created SQL code
-- First, add the static code at the start:
declare #SQL varchar(max) =
' SELECT t.[SurveyID]
, x.question
, x.Answer
FROM tbl t
CROSS APPLY
(
'
-- This syntax will add to the variable for every row in the query results; it's a little like looping over all the rows.
select #SQL +=
'select ' + cast(C.column_id as varchar)
+ ' as QuestionNumber, ''' + C.name
+ ''' as Question , t.' + C.name
+ ' As Answer union all
'
from sys.columns C
inner join sys.tables T on C.object_id=T.object_id
where T.name = 'MySurveyTable'
-- Remove final "union all", add closing bracket and alias
set #SQL = left(#SQL,len(#SQL)-10) + ') x'
print #SQL
-- To also execute (run) the dynamically-generated SQL
-- and get your desired row-based output all at the same time,
-- use the EXECUTE keyword (EXEC for short)
exec #SQL
A similar approach could be used to dynamically write SQL for the UNPIVOT approach.