Dynamic sql and spexecutesql behavior - sql-server

Question

I have a table like
c1 c2 c3 c4
-----------------
2 1 7 13
9 2 8 14
1 3 9 15
5 4 10 16
2 5 11 17
11 6 12 18
As in general I would not know the number of columns (in the code #d here 4) to get a string in the form:
2,9,1,5,2,11, 1,2,3,4,5,6, 7,8,9,10,11,12, 13,14,15,16,17,18
To do so I am doing:
DECLARE #d INT
,#counterI INT
,#template AS nvarchar(max)
SET #d = 4;
SET #counterI = 1;
Set #template = 'SELECT STUFF(
( SELECT '','' + CAST([col] AS VARCHAR) FROM (';
WHILE (#counterI < #d) BEGIN
SET #template += ' SELECT [c'+CAST(#counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] UNION ALL ';
SET #counterI = #counterI + 1;
END
Set #template += ' SELECT [c'+CAST(#counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] '
Set #template += ') alldata FOR XML PATH('''') ) , 1 , 1 , '''' )';
declare #CommaString varchar(max)
set #CommaString = ''
exec sp_executesql #template, N'#CommaString varchar(max) out', #CommaString out
So if I do
select #CommaString;
Why is not #CommaString at the moment of selecting it returning the string if when doing the sp_executesql it is printing it right?

Answer 1

I may be missing something about how sp_executesql works, but don't you need something like 'SELECT #CommaString = ...' in #template, so that it assigns the comma string to the out parameter?
Just to clarify, I think you need something like:
DECLARE #d INT
,#counterI INT
,#template AS nvarchar(max)
SET #d = 4;
SET #counterI = 1;
Set #template = 'SELECT #CommaString = STUFF(
( SELECT '','' + CAST([col] AS VARCHAR) FROM (';
WHILE (#counterI < #d) BEGIN
SET #template += ' SELECT [c'+CAST(#counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] UNION ALL ';
SET #counterI = #counterI + 1;
END
Set #template += ' SELECT [c'+CAST(#counterI-1 AS VARCHAR)+'] AS col FROM [MyTable] '
Set #template += ') alldata FOR XML PATH('''') ) , 1 , 1 , '''' )';
declare #CommaString varchar(max)
set #CommaString = ''
exec sp_executesql #template, N'#CommaString varchar(max) out', #CommaString = #CommaString out
As a simpler example, something like this is perhaps easier to read/see what I mean:
declare #CommaString varchar(max)
set #CommaString = ''
exec sp_executesql 'SELECT #CommaString = ''1,2,3''', '#CommaString varchar(max) out', #CommaString = #CommaString out
Incidentally, I've usually seen this kind of thing for string concatenation:
DECLARE #MyString varchar(max)
SET #MyString = ''
SELECT #MyString = #MyString + ',' + MyColumn
FROM MyTable