Nice one but it did not handle null values you can change it as following to handle null values
create table #TABLE_1 (COLUMN_1 varchar(5))
insert into #TABLE_1
select 'A'
union all
select 'B'
union all
select 'C'
union all
select 'D'
union all
select 'E'
union all
select null
declare @list varchar(max)
select @list=COALESCE(@list+', ','')+ isnull(COLUMN_1,'NULL') from #TABLE_1
select @list
drop table #TABLE_1
This is just a update in existing trick..
Thanks