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

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

For MySQL:
SELECT id, GROUP_CONCAT(displayname) FROM tableName GROUP BY id

Refer: http://www.sqlines.com/mysql/functions/group_concat

SQL Server 2017+ and SQL Azure: STRING_AGG

Starting with the next version of SQL Server, we can finally concatenate across rows without having to resort to any variable or XML witchery.

STRING_AGG (Transact-SQL)

SELECT ID, STRING_AGG(DisplayName, ', ') AS DisplayNames
FROM TableName
GROUP BY ID

and in case of oracle database

select id, 
       listagg(displayname, ',') within group (order by displayname) as names
from test
group by id

to change the separator use

SELECT id, GROUP_CONCAT(displayname SEPARATOR ';') FROM tableName GROUP BY id

this will change separator from comma to semicolon :)

Thank you all,

SELECT Distinct
    t1.ID,
    MAX(STUFF(t2.x_id,1,1,'')) AS DisplayName
FROM Table t1
CROSS apply(
    SELECT Distinct ', ' + SUBSTRING(t2.DisplayName,1,2)
    FROM Table t2
    WHERE t2.ID = t1.ID AND t2.DisplayName > ''
    FOR xml PATH('')
) AS t2 (x_id)
GROUP BY
    t1.ID
order by 1
GO

In Oracle SQL, you can use LISTAGG .

SELECT ID, LISTAGG(DisplayName, ', ') as all_agg
FROM TABLE_NAME GROUP BY ID;