Merge multiple results to one Column - sql-server

Question

I would like to create a select statement to merge multiple results into one rot
I was able to realized what I want via PHP but i think it should be possible to work directly with Sql.
Notes
| ID | Name |
|----|------|
| 2 | Test |
| 3 | Test |
EditorAssignment
| UserID | NoteID |
|--------|--------|
| 1 | 2 |
| 2 | 2 |
UserList
| ID | username |
|----|----------|
| 1 | testuser |
| 2 | bUser |
| 3 | cUser |
I would like to have the following Result:
| NoteID | User |
|--------|-----------------|
| 2 | testuser, bUser |
| 3 | cUser |
How can i create a select like that in SQL2012?

Answer 1

Before SQL Server 2017 (which has STRING_AGG aggregate function for this), you need to use the famous FOR XML PATH correlated subquery:
SetUp:
DECLARE #Notes TABLE (ID INT, Name VARCHAR(10))
INSERT INTO #Notes (ID, Name)
VALUES (2, 'Test'), (3, 'Test')
DECLARE #EditorAssignment TABLE (UserID INT, NoteID INT)
INSERT INTO #EditorAssignment (UserID, NoteID)
VALUES (1, 2), (2, 2),
(3, 3) -- Added missing row here
DECLARE #UserList TABLE (ID INT, username VARCHAR(100))
INSERT INTO #UserList (ID, username)
VALUES (1, 'testuser'), (2, 'bUser'), (3, 'cUser')
Query:
SELECT
NoteID = N.ID,
[User] = STUFF(
(
SELECT
', ' + U.username
FROM
#EditorAssignment AS E
INNER JOIN #UserList AS U ON E.UserID = U.ID
WHERE
N.ID = E.NoteID -- Link the outmost Notes' note with the inner EditorAssignment (correlated subquery)
FOR XML
PATH ('') -- FOR XML PATH('') makes the SELECT return a string value, not a result set
),
1,
2,
'')
FROM
#Notes AS N
Results:
NoteID User
2 testuser, bUser
3 cUser
The subquery generates a string with each username for the related assignment for each note.
The STUFF function is used just to replace the first ', ', since the string is build with a leading comma and a space. This is why the parameters of the STUFF are 1 (first position of the string), 2 (amount of places to remove, the comma and the space), and '' (replacement for those characters).
I'm guessing that you have a row missing for the EditorAssignment table that holds the values 3, 3 on your example.