Should never happen ... <\THUD!>
But because this is a trigger and I want it to work in all instances, process all rows in INSERTED I opted for an Outer Join.
The SET statement has a COALESCE on it which will take the "best value" from either OUTER JOIN table or, if none, then leave the value as it was provided. This is some very "Belt & Braces" code.
I'm busy trying to figure out how I can get a Query Plan from the Trigger. Being an old dinosaur, and preferring to view it in TEXT I normally use
SET SHOWPLAN_TEXT ON
I can see it (in Result #2, in SSMS ... but all that "percentages" stuff and MouseOver to see a bit more is a PITA to me ... Perhaps I can save it to a .sqlplan fie and then examine that with something or other?