Here is one way:

declare @t table (i int, c char(1))

insert into @t (i,c) values (1,'a'),(1,'b'),(1,'c'),(2,'a'),(2,'b')

select * from @t;

With MyCTE as (

select distinct i, (select t2.c + ',' from @t as t2 where t1.i = t2.i for xml path('')) as Letters

from @t t1)

select i, left(Letters, len(Letters) - 1)

from MyCTE

go

Adi