I got it wrong because I misinterpreted this line:
When you specify a PRIMARY KEY constraint for a table, the Database Engine enforces data uniqueness by creating a unique index for the primary key columns.
an index but doesn't use the PRIMARY KEY as that index. Annoying, but my fault.
On two occasions I have been asked, "Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?" ... I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question.
—Charles Babbage, Passages from the Life of a Philosopher How to post a question to get the most help