WITH numbered AS (
SELECT
ID, Seq, Amt,
SeqGroup = ROW_NUMBER() OVER (PARTITION BY ID ORDER BY Seq) - Seq
FROM atable
)
SELECT
ID,
Start = MIN(Seq),
[End] = MAX(Seq),
TotalAmt = SUM(Amt)
FROM numbered
GROUP BY ID, SeqGroup
ORDER BY ID, Start
;
