So your jobs table has no primary key? What column(s) are used to indicate that an aggregate row belongs to a job row? No offense mate but your table design...

You have to get the scalar value logic out of your head to start thinking about this as a set. You could use MERGE but separate insert and update statements...

Your trigger has a MAJOR flaw. It assumes there will only ever be a single row in inserted. In sql server triggers fire once per operation, not once per row....