A program error (by yours truly) caused several rows to be duplicated in a client's table.

I needed a script to delete the dupes so I searched and found this little jewel.

Other scripts involve temporary tables, etc but this simple script works like a charm.

My newbie question is .. I thought you had to specify all the group by elements in the query result.
Obviously you don't.
What am I missing?

DELETE
FROM MyTable
WHERE ID NOT IN
(
SELECT MAX(ID)
FROM MyTable
GROUP BY DuplicateColumn1, DuplicateColumn2, DuplicateColumn3)