Вот, в QA наваял, вроде работает. Можно, наверное в 2000 оформить в виде функции, не знаю - у меня 7.0.

declare @Str varchar(800)
declare @Res varchar(800)
declare @TmStr varchar(800)
declare @Delta int
declare @Min int
declare @Max int

set @Str='21,22,23,25,27,28,29,30,31,50'
set @Res=''

create table #Tmp(id int IDENTITY ,f int)
set @TmStr='insert #Tmp (f) select f from ( select f='+
REPLACE(@Str,',',' union select f=')+') as s order by f'
Exec(@TmStr)

set @Delta= (select top 1 f-id
from #Tmp
order by id)

while (select count (*) from #Tmp)!=0
begin --1
set @Delta= (select top 1 f-id
from #Tmp
order by id)

select @Min=min(f), @Max=max(f)
from #Tmp
where id=f-@Delta

delete from #Tmp
where id=f-@Delta

if @Max=@Min
set @Res=@Res+RTRIM(LTRIM(STR(@Max)))+','
else
set @Res=@Res+RTRIM(LTRIM(STR(@Min)))+'-'+RTRIM(LTRIM(STR(@Max)))+','
end --1

set @Res=SUBSTRING(@Res,1,LEN(@Res)-1)
select @Res