DECLARE @t TABLE (GroupId INT, Subgroup INT)

INSERT INTO @t
             SELECT 1, 11
   UNION ALL SELECT 1, 12
   UNION ALL SELECT 1, 13
   UNION ALL SELECT 2, 21
   UNION ALL SELECT 2, 22
   UNION ALL SELECT 2, 23
   UNION ALL SELECT 3, 31
   UNION ALL SELECT 3, 32

SELECT
      GroupId,
      MIN(Subgroup)
   FROM @t
   GROUP BY GroupId

;with CTE as
	(select GroupNumber, SubGroup, 
	row_number() over (partition by GroupNumber order by SubGroup) as Row
	from dbo.MyTable)
select GroupNumber, SubGroup
from CTE
where Row = 1;