Datomic queries against Clojure collections

datomic_clojure_collections.clj

;; Datomic example code
(use '[datomic.api :only (db q) :as d])

;; ?answer binds a scalar
(q '[:find ?answer :in ?answer]
   42)

;; of course you can bind more than one of anything
(q '[:find ?last ?first :in ?last ?first]
   "Doe" "John")

;; [?last ?first] binds a tuple
(q '[:find ?last ?first :in [?last ?first]]
   ["Doe" "John"])

;; [?first ...] binds a collection
(q '[:find ?first
     :in [?first ...]]
   ["John" "Jane" "Phineas"])

;; [[?first ?last]] binds a relation
(q '[:find ?first
     :in [[?first ?last]]]
   [["John" "Doe"]
    ["Jane" "Doe"]])

;; a database binding name starts with $ instead of ?
;; any relation with 4-tuples E/A/V/T can act as a database
;; so in Datomic, you can mock a database with a list of lists
(q '[:find ?first
     :in $db
     :where [$db _ :firstName ?first]]
   [[1 :firstName "John"]])

;; same as previous, but omit $db for single-database query
;; any relation with 4-tuples eavt can act as a database
(q '[:find ?first
     :where [_ :firstName ?first]]
   [[1 :firstName "John" 42]
    [1 :lastName "Doe" 42]])

;; simple in-memory join, two tuple bindings
(q '[:find ?first ?height
     :in [?last ?first ?email] [?email ?height]]
   ["Doe" "John" "[email protected]"]
   ["[email protected]" 71])

;; simple in-memory join, two relation bindings
;; see next example for a faster approach
(q '[:find ?first ?height
     :in [[?last ?first ?email]] [[?email ?height]]]
   [["Doe" "John" "[email protected]"]
    ["Doe" "Jane" "[email protected]"]]
   [["[email protected]" 73]
    ["[email protected]" 71]])

;; same as previous example, but with database expressions
;; runs faster than relation bindings (as of July 2012) 
(q '[:find ?first ?height
     :in $a $b
     :where [$a ?last ?first ?email]
            [$b ?email ?height]]
   [["Doe" "John" "[email protected]"]
    ["Doe" "Jane" "[email protected]"]]
   [["[email protected]" 73]
    ["[email protected]" 71]])

;; simple in-memory join, two database bindings
(q '[:find ?first ?height
     :in $db1 $db2
     :where [$db1 ?e1 :firstName ?first]
            [$db1 ?e1 :email ?email]
            [$db2 ?e2 :email ?email]
            [$db2 ?e2 :height ?height]]
   [[1 :firstName "John"]
    [1 :email  "[email protected]"]
    [2 :firstName "Jane"]
    [2 :email "[email protected]"]]
   [[100 :email "[email protected]"]
    [100 :height 73]
    [101 :email "[email protected]"]
    [101 :height 71]])

;; compare to http://stackoverflow.com/questions/3717939/iterating-and-processing-an-arraylist
(q '[:find ?car ?speed
     :in [[?car ?speed]]
     :where [(> ?speed 100)]]
   [["Stock" 225]
    ["Spud" 80]
    ["Rocket" 400]
    ["Stock" 225]
    ["Clunker" 40]])

;; compare to http://stackoverflow.com/questions/109383/how-to-sort-a-mapkey-value-on-the-values-in-java
(->> (q '[:find ?k ?v
          :in [[?k ?v] ...]]
        {:D 67.3 :A 99.5 :B 67.4 :C 67.5})
     (sort-by second))