String Aggregate Concatenation

Question

Link on aggregate concatenation with variable:
CREATE FUNCTION CSVLanguage (@Title as varchar(50))
RETURNS varchar(1000)
AS
BEGIN
DECLARE @Sa varchar(1000)
SELECT @Sa=@Sa + language + ','
    FROM books
    WHERE title=@Title
RETURN (@Sa)
END
I have seen the above concatenation method zillion times, kind of thought it is a valid method. In fact I used it many times, and always worked.
Erland Sommarskog has pointed out in the following (unrelated) thread that the behaviour is undefined in such an aggregate concatenation:
http://social.msdn.microsoft.com/Forums/en/transactsql/thread/31804823-6e5e-4e2d-9729-330d12ccb880
Erland provided the following kb article for explanation:
http://support.microsoft.com/kb/287515
Since T-SQL does not (yet!) have a built-in function for aggregate concatenation, a T-SQL - XML hybrid method popped up with SQL Server 2005 ( I also used this many times to satisfaction):
SELECT DISTINCT id,
 SUBSTRING(
 (SELECT ','+name AS [text()]
   FROM t1
   WHERE id = T.id
   ORDER BY name
   FOR XML PATH( '' )
 ), 2,100) AS concat
 FROM t1 AS T;
As noted in the link above, this method has some issues as well.
-- DEMO: with "<" as part of the string, wrong concatenation result
SELECT ID=productID/100, Name='<'+Name
INTO t1
FROM AdventureWorks2008.Production.Product
go
SELECT DISTINCT id,
SUBSTRING(
(SELECT ','+name AS [text()]
FROM t1
WHERE id = T.id
ORDER BY name
FOR XML PATH( '' )
), 2,100) AS concat
FROM t1 AS T;
GO
id concat
0 &lt;Adjustable Race,&lt;BB Ball Bearing,&lt;Bearing Ball,&lt;Headset Ball Bearings
3 &lt;Blade,&lt;Chain Stays,&lt;Chainring,&lt;Chainring Bolts,&lt;Chainring Nut,&lt;Crown Race,&lt;Dec
DROP TABLE t1;
So what is the real solution to aggregate concatenate?
Kalman Toth SQL SERVER 2012 & BI TRAINING
New Book: Beginner Database Design & SQL Programming Using Microsoft SQL Server 2012

Answer 1

an alternative query with hexBinary provides correct result
declare @sample table
id int,
data varchar(100)
insert @sample
values (1, 'Peso & Performance SQL>'),
(1, 'MVP'),
(2, 'Need help <' + CHAR(2) + '? /> + ' + CHAR(31) + ' f'),
(2, 'With XML string concatenation ?')
select i.id,
f.content
from (
select id
from @sample
group by id
) as i
outer apply (
select stuff(convert(varchar(max), convert(xml,'').value('xs:hexBinary(sql:column("w4.data"))', 'varbinary(max)')),1,2,'')
as Content
from (select convert(varchar(max),w3.data)
from (
select convert(xml,'').value('xs:hexBinary(sql:column("w2.data"))', 'varchar(max)')
from (select convert(varbinary(max), ', ' + w1.data)
from @Sample w1
where w1.ID = i.ID
) w2 (data)
for xml path(''),type
) w3 (data)
) w4 (data)
) f