delete from table
where exist (
select 1
from table t
join (select group, value, min(len(prefix)) lprfx
from table
group by group, value ) p
on t.group = p.group
and t.value = p.value
and len(t.prefix)> p. lprfx
)
ha tobb azonos min. hosszu prefix van meghagyja mindet. (nem tesztteltem, fejbol irtam)