Subqueries

[quoll]

Subqueries are currently possible by piping the results of one query into the next:

(->>
  (q ‘[:find ?person (count ?child) :where [?person :children ?children] [?children :tg/contains ?child]] the-db)
  (q ‘[:find ?city
       :in $ [[?person ?count-child]]
       :where [(> ?count-child 3)] [?person :address ?address] [?address :city ?city]] the-db))

This works efficiently, as it is only 2 queries, with the bindings of the first query being lazily consumed by the second.

However, this is difficult to follow. After some discussion with @marioaquino we're thinking that the following may work as a syntax:

(q ‘[:find ?city
     :in $ [:find ?person (count ?child) :as ?cc :where [?person :children ?children] [?children :tg/contains ?child]]
     :where [(> ?cc 3)] [?person :address ?address] [?address :city ?city]] the-db)

Things to note:

• Thinking that the inner query should not have an :in clause. It must inherit outer bindings from the outer query (with the subquery removed).
• Alternatively, perhaps the inner query could have an :in but only to allow another inner query.
• An inner query must be the final argument to an :in clause.
• Arguments to q must match all non-query args to the :in clause.
• By default, aggregate bindings are labeled as: the aggregate function name; a hyphen; the variable name the aggregate is calculated from. So (count ?child) is labeled: ?count-child. The :as keyword would rename this.

[quoll]

Depends on issue #192