;with a as (select * from (values ('1_DK', 'блаблабла'),
('1_DK', 'трыы'),
('1_SEL', 'ууууууууууу'),
('1_SEL', 'юююююююю'),
('2_SEL', 'ццццц'),
('2_SEL', 'hhhhhhhh')) as t (id, name))
, b as (select distinct substring(id,1,charindex('_',id)-1) as prefix
, substring(id,charindex('_',id)+1,DATALENGTH(id)-charindex('_',id)) as suffix
from a)
select prefix+'_'+suffix as id from b bb
where suffix = 'DK' or not exists (select 1 from b bb1 where bb1.suffix='DK' and bb1.prefix=bb.prefix)