Function with XML path doesnt work - sql-server

Question

I want to write a function that concat field from different row and group by id.
When I execute my code in TSql is work fine, But when I execute it as a function it returns wrong data, Why?
CREATE TABLE YourTable ([ID] INT, [Name] CHAR(1), [Value] INT)
INSERT INTO YourTable ([ID],[Name],[Value]) VALUES (1,'A',4)
INSERT INTO YourTable ([ID],[Name],[Value]) VALUES (1,'B',8)
INSERT INTO YourTable ([ID],[Name],[Value]) VALUES (2,'C',9)
INSERT INTO YourTable ([ID],[Name],[Value]) VALUES (3,'d',9)
INSERT INTO YourTable ([ID],[Name],[Value]) VALUES (3,'u',9)
SELECT
[ID],
STUFF((
SELECT ', ' + [Name]
FROM YourTable
WHERE (ID = Results.ID)
FOR XML PATH(''),TYPE).value('(./text())[1]','VARCHAR(MAX)'),1,2,'') AS NameValues
FROM YourTable Results
GROUP BY id;
create FUNCTION dbo.CONCAT_String_group_by_id (#id int , #name varchar
(100) )
RETURNS TABLE AS return
WITH cte AS(
SELECT #id id ,#name name
FROM (VALUES(0)) a(m))
SELECT
[ID],
STUFF((
SELECT ', ' + [Name]
FROM cte
WHERE (ID = Results.ID)
FOR XML PATH(''),TYPE).value('(./text())[1]','VARCHAR(MAX)'),1,2,'') AS NameValues
FROM cte Results
GROUP BY id
SELECT n.*
FROM YourTable CROSS APPLY dbo.CONCAT_String_group_by_id (id,name) n
First query Output:
ID NameValues
1 A, B
2 C
3 d, u
Second query Output:
ID NameValues
1 A
1 B
2 C
3 d
3 u

Answer 1

Your function wont work ,because Cross apply will be executed for each row of outer query once and outputs the result
In your first query,you are evaluating all the data at once and logical order of execution goes like below
query:
SELECT
[ID],
STUFF((
SELECT ', ' + [Name]
FROM YourTable
WHERE (ID = Results.ID)
FOR XML PATH(''),TYPE).value('(./text())[1]','VARCHAR(MAX)'),1,2,'') AS NameValues
FROM YourTable Results
GROUP BY id;
1.group by id from your table
2.Do a concat of all IDs Present using XML
In your second query ,you are using cross apply to pass one row at a time and it will have only one row to concat ,so your output varies

Answer 2

What about this function:
CREATE FUNCTION dbo.CONCAT_String_group_by_id (#id int)
RETURNS TABLE AS return
WITH cte AS(
SELECT *
FROM YourTable
WHERE id = #id
)
SELECT
[ID],
STUFF((
SELECT ', ' + [Name]
FROM cte
WHERE (ID = Results.ID)
FOR XML PATH(''),TYPE).value('(./text())[1]','VARCHAR(MAX)'),1,2,'') AS NameValues
FROM cte Results
GROUP BY id
In your function you have just one row in your CTE (WITH) so always you will have one letter in NameValues.