--Working table: when individual items are inserted,
--trigger concatenates them to a list
create table Tstr (
  s varchar(8000)
)
go

create trigger Tstr_ins
on Tstr for insert as
if (select count(*) from inserted) % 2 = 0 begin
  truncate table Tstr
  insert into Tstr
  select max(I1.s)+', '+I2.s
  from inserted I1 join inserted I2
  on I1.s < I2.s
  group by I2.s
  having count(I1.s)%2=1
end else
if (select count(*) from inserted) > 1 begin
  truncate table Tstr
  insert into Tstr
  select max(I1.s)+', '+I2.s
  from inserted I1 join inserted I2
  on I1.s < I2.s
  group by I2.s
  having count(I1.s)%2=1
  union all
  select max(s) from inserted
end

go
--example 1
  insert into Tstr
  select distinct top 21 customerid from northwind..orders
  select s from Tstr
  truncate table Tstr

--example 2
  insert into Tstr
  select au_id from pubs..authors
  select s from Tstr
  truncate table Tstr
go

drop table Tstr