You can use below query -

create table #tmp1

(col1 varchar(1)

,col2 varchar(1)

)

insert into #tmp1

select 'a','l' UNION ALL

select 'b','p' UNION ALL

select 'a','m' UNION ALL

select 'a','n' UNION ALL

select 'b','q' UNION ALL

select 'x','y'

SELECT col1

,STUFF((SELECT ', ' + CAST(col2 AS VARCHAR(10)) [text()]

FROM #tmp1

WHERE col1 = t.col1

FOR XML PATH(''), TYPE)

.value('.','NVARCHAR(MAX)'),1,2,' ') List_Output

FROM #tmp1 t

GROUP BY col1