T-SQL function with dynamic SELECT (not possible) - solved with procedure instead - function

Question

I've managed to use EXEC sp_executesql in a one off statement to do a dynamic lookup, but am unable to adjust the code to create a function since EXEC is not allowed in functions. It works in procedures and I've managed to get output via PRINT for a single lookup by using a temporary table, but really that was just me struggling to find a workaround. Ideally I'd like to be able to create a scalar-value function.
The reason that I need a dynamic lookup is because the column name is stored in another table.
Here's a quick breakdown of the tables:
Questions:
Columns: Q_Group, Q_Nbr, Question_Desc, Data_Field
Sample data: 'R3', 5, 'Do you have any allergies?', 'TXT_04'
Responses:
Columns: Order_Nbr, Q_Group, TXT_01, TXT_02, TXT_03, TXT_04, etc.
Data: 999, 'R3', 'blah', 'blah', 'blah', 'NO'
Orders will be assigned a particular set of questions 'Q_Group' and often a particular question will be the same across various different sets of questions. The problem is that when the set/groups of questions were set up, the questions may not have been added in the same order, and thus the responses go into different columns.
So here's where I'm at...
I can get 'TXT_04' from the Data_Field column in Questions and use EXEC sp_executesql to do a lookup for a single order, but am struggling to find a way to accomplish this as a function of some sort.
DECLARE #col_name VARCHAR(6)
DECLARE #sql VARCHAR(100)
SET #col_name = SELECT Data_Field FROM QUESTIONS WHERE Q_Group = 'R3'
AND Question_Desc = 'Do you have any allergies?'
SET #sql = 'SELECT ' + #col_name + ' FROM RESPONSES WHERE Order_Nbr = 999'
EXEC sp_executesql #sql
I'm just at a loss as to how this could be incorporated into a function so that I could get responses for several orders in a result set. Any workarounds possible? Maybe I'm totally off base using EXEC sp_executesql?
Thanks.
Edit...
Okay, I've changed the title to reflect that I'm going to consider this solved with a procedure instead of a function, as it ended up getting the output that I wanted. Which was a table with all of the corresponding responses.
Here's the code that I settled on. I decided to use LIKE to match the Question_Desc instead of equals, and then included the Question_Desc in the results, so that it could be used a bit more broadly. Thankfully it's pretty quick to run currently. Although that could always change as the database grows!
CREATE PROCEDURE get_all_responses (#question_txt VARCHAR(255))
AS
DECLARE #response_col VARCHAR(35)
DECLARE #t TABLE (order_nbr int, question_txt VARCHAR(255), response_col VARCHAR(35), response VARCHAR(255))
DECLARE #i TABLE (id INT PRIMARY KEY IDENTITY(1,1), response_col VARCHAR(35))
DECLARE #u TABLE (order_nbr int, response VARCHAR(255))
DECLARE #sql VARCHAR(200)
INSERT #t
SELECT Order_Nbr, Question_Desc, Data_Field, NULL
FROM Responses
JOIN (
SELECT Q_Group, Question_Desc, Data_Field
FROM Questions
WHERE Question_Desc LIKE #question_txt
) #Q ON Q_Group = #Q.Q_Group
WHERE Q_Group <> '0'
ORDER BY Data_Field, Order_Nbr
-- Stop if no results found and return empty result set
IF (SELECT COUNT(*) FROM #t) = 0
BEGIN
SELECT order_nbr, question_txt, response FROM #t
RETURN
END
INSERT #i SELECT response_col FROM #t GROUP BY response_col
DECLARE #row_nbr int
DECLARE #last_row int
SET #row_nbr = 1
SET #last_row = (SELECT COUNT(*) FROM #i)
-- Iterate through each Data_Field found
WHILE #row_nbr <= #last_row
BEGIN
SET #response_col = (SELECT response_col FROM #i WHERE id = #row_nbr)
SET #sql = 'SELECT Order_Nbr, ' + #response_col + ' FROM Responses WHERE NullIf(' + #response_col + ','''') IS NOT NULL'
INSERT INTO #u
EXEC (#sql)
UPDATE #t
SET response = y.response
FROM #t AS x
INNER JOIN #u AS y ON x.order_nbr = y.order_nbr
SET #row_nbr = #row_nbr + 1
END
-- Remove results with no responses
DELETE FROM #t WHERE response IS NULL
SELECT order_nbr, question_txt, response FROM #t
RETURN

Answer 1

You will not be able to execute dynamic SQL from within a function but you could do this with a stored procedure and capture the output.
DECLARE #col_name VARCHAR(6), #param NVARCHAR(50), #myReturnValue VARCHAR(50)
SET #param = N'#result VARCHAR(50) OUTPUT'
DECLARE #sql VARCHAR(100)
SET #col_name = SELECT Data_Field FROM QUESTIONS WHERE Q_Group = 'R3'
AND Question_Desc = 'Do you have any allergies?'
SET #sql = 'SELECT #result = ' + #col_name + ' FROM RESPONSES WHERE Order_Nbr = 999'
EXEC sp_executesql #sql, #param, #result = #myReturnValue output
--manipulate value here
print #myReturnValue
You could also create a temp table and do an insert into from exec sp_executesql.