Thanks, David and Tim.
From David's reply, it seems that I can create a trivial project as long as I know the name of the one and only one "holder field" from the source.
From Tim's reply, it seems that there is no identity expression. That is going to make logical expressions more difficult to work with, especially since Drill is dynamically typed. (Just as arithmetic was difficult before someone invented zero.)
In the SQL-to-LE translator, I think I can glean the holder expression from the context, so I should be able to use David's approach.