tsql - Convert multiple rows into one with comma as separator

If I issue SELECT username FROM Users I get this result:

username
--------
Paul
John
Mary

but what I really need is one row with all the values separated by comma, like this:

Paul, John, Mary

How do I do this?

Answer

This should work for you. Tested all the way back to SQL 2000.

create table #user (username varchar(25))


insert into #user (username) values ('Paul')
insert into #user (username) values ('John')
insert into #user (username) values ('Mary')

declare @tmp varchar(250)
SET @tmp = ''
select @tmp = @tmp + username + ', ' from #user

select SUBSTRING(@tmp, 0, LEN(@tmp))