Vad sägs om att använda ROW_NUMBER
:
WITH Cte AS(
SELECT *,
RnAsc = ROW_NUMBER() OVER(PARTITION BY [group] ORDER BY val),
RnDesc = ROW_NUMBER() OVER(PARTITION BY [group] ORDER BY val DESC)
FROM tbl
)
SELECT
id, [group], val, start, [end]
FROM Cte
WHERE
RnAsc = 1 OR RnDesc = 1
ORDER BY [group], val