Great job!

And here is a small suggestion, you might want to change the name of output column "unique" in Q3 because.

  *expression* as unique --does work and is recommended answer
  *expression* unique --fail because of use of a restricted word
  *expression* "unique" --work because of double quote

Using a restricted word that need quotation in some context is globally discouraged and in this context can keep someone stuck independently of the problem at stack.

PS: also apply to "position" in Q4 maybe thats intentional?