How to concatenate many rows with same id in sql?

DECLARE @t TABLE
(
    ID INT,
    DisplayName VARCHAR(50)
)
INSERT INTO @t (ID, DisplayName)
VALUES
    (1 , 'Editor'),
    (1 , 'Reviewer'),
    (7 , 'EIC'),
    (7 , 'Editor'),
    (7 , 'Reviewer'),
    (7 , 'Editor'),
    (19, 'EIC'),
    (19, 'Editor'),
    (19, 'Reviewer')

SELECT *, STUFF((
            SELECT DISTINCT ', ' + DisplayName
            FROM @t
            WHERE ID = t.ID
            FOR XML PATH('')), 1, 2, '')
FROM (
    SELECT DISTINCT ID
    FROM @t
) t

Output -

----------- ------------------------
1           Editor, Reviewer
7           Editor, EIC, Reviewer
19          Editor, EIC, Reviewer

My post about string aggregation:

http://www.codeproject.com/Articles/691102/String-Aggregation-in-the-World-of-SQL-Server

In SQL-Server you can do it in the following:

QUERY

SELECT id, displayname = 
    STUFF((SELECT DISTINCT ', ' + displayname
           FROM #t b 
           WHERE b.id = a.id 
          FOR XML PATH('')), 1, 2, '')
FROM #t a
GROUP BY id

TEST DATA

create table #t 
(
id int,
displayname nvarchar(max)
)

insert into #t values    
 (1 ,'Editor')
,(1 ,'Reviewer')
,(7 ,'EIC')
,(7 ,'Editor')
,(7 ,'Reviewer')
,(7 ,'Editor')
,(19,'EIC')
,(19,'Editor')
,(19,'Reviewer')

OUTPUT

id  displayname
1   Editor, Reviewer
7   Editor, EIC, Reviewer
19  Editor, EIC, Reviewer