Good question and good explanation - and thanks especially for the link to a late draft of the SQL 2003 standard.

It was a bit hard for a Friday question, just when people are getting ready to relax for the weekend (careless scheduling by Steve there, or did he do it on purpose?). I spent a long time racking my brain to try to remember which was the fourth box to tick - after I had ticked three boxes all the others seemed as if they ought to be left unticked; and then I clicked that only UPDATE was allowed in WHEN MATCHED, not DELETE, so as I had already ticked WHEN NOT MATCHED BY SOURCE I logically had to tick DELETE as my fourth choice.

Definitely good excercise for the grey matter.