create table tb(id int, value varchar(10))

insert into tb values(1,'aa')

insert into tb values(1,'bb')

insert into tb values(2,'aaa')

insert into tb values(2,'bbb')

insert into tb values(2,'ccc')

GO

SELECT id,value = stuff ((SELECT     ',' + value FROM   tb AS t
    WHERE     t .id = tb.id FOR xml path('')), 1, 1, '')
FROM  tb
GROUP BY id