Thanks for the answer!

It works fine - would just have to get it into an UPDATE Statement - got a very different and simpler solution though from stackexchange, which even fixes my problem when it comes to having multiple Courses with the same Priorities (for anyone interested):

update D1

set CourseDate = dateadd(day, (select isnull(sum(D2.Delay), 0)

from dbo.set_dates as D2

where D1.order_id = D2.order_id and

D1.Priority > D2.Priority), D1.st_date)

from set_dates as D1

Thanks again though and I'll mark your answer 🙂 ...