For those interested in database query languages, it is worth knowing about Datalog, the query language behind Datomic, XTDB and Datahike: http://www.learndatalogtoday.org/

E.g. a parameterised aggregate query that retrieves the name and average rating of a film starring cast members whose names match the input names:

    [:find ?name (avg ?rating)
     :in $ [?name ...] [[?title ?rating]]
     :where
     [?p :person/name ?name]
     [?m :movie/cast ?p]
     [?m :movie/title ?title]]

To reveal the answer, click on tab labelled "3" and then "I give up!": http://www.learndatalogtoday.org/chapter/7