-- 1 Group by groupcol,
-- 2 Sort by sortcol
-- 3 Add numeration for every group 

select table.*,
row_number() over (partition by table.groupcol order by sortcol) as rm -- set number for each group
from table;

-- leave only one row for each group

select * from
  (select table.*,
  row_number() over (partition by table.groupcol order by sortcol) as rm -- set number for each group
  from table)
where rm = 1;