Prøv dette
SELECT 100+ Sum(CASE WHEN name = 'a' THEN qty ELSE 0 END)OVER(ORDER BY itemorder DESC) as modifyed_sum,
qty,
name,
itemorder
FROM Yourtable
ORDER BY itemorder ASC
En anden måde
SELECT 100 + (SELECT Sum(CASE WHEN b.name = 'a' THEN b.qty ELSE 0 END)
FROM yourtable b
WHERE a.itemorder <= b.itemorder),
qty,
name,
itemorder
FROM yourtable a
ORDER BY itemorder ASC