@Charlieface I don't believe you can express it directly. Aside from relying on the output order of a FULL MERGE JOIN, you might be able to express it as dbfiddle.uk/_ddCBZLv
Assuming your main concern is avoiding a sort while guaranteeing order
OMG - I'll have to investigate further - you've actually read the heretical texts of the PostgreSQL Ayatollahs! Fair play for diversity of knowledge - you'll probably be condemned to a life of MySQL db maintenance for your pains! :-)
@PaulWhite Something like that was tumbling around in my head but couldn't put my finger on it. Annoying you can't specify it explicitly. Logically the compiler should be able to infer ISNULL(id1, id2) to mean the same thing.