[petergeoghegan]

What most users want is a statement that allows them to UPSERT, with reasonable guarantees around that never throwing a duplicate violation error, and never deadlocking (at least not in any novel way caused by implementation details). Something practical.

That's not the kind of thing that the standard takes a position on, for a variety of reasons. In general, it says very little about concurrency control/isolation. And what little is does say is all very abstract.

For whatever reason, a lot of people believe that MERGE promises something over and above what using multiple INSERT, UPDATE, and DELETE statements will promise. But that simply isn't true. The standard doesn't say anything that even vaguely suggests otherwise. Same is true of both the SQL Server MERGE docs, and the Oracle MERGE docs.