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basic_queries.md

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Basic Queries

Queries are executed via d/q. It takes a quoted vector '[...] containing the query.

A query is built up of the following parts:

  • :find
  • :in (optional: to pass query parameters)
  • :where (optional: to define constraints)

Let's make a query to find the :name and :age of any entity who has the alias (:aka) of "Maks Otto von Stirlitz".

The result is: ["Maksim" 45]

  (d/q '[ :find  ?n ?a
          :where [?e :aka "Maks Otto von Stirlitz"]
                 [?e :name ?n]
                 [?e :age  ?a] ]
       @conn))

;; => #{ ["Maksim" 45] }

The symbol _ can be used as a wildcard for the parts of the data pattern that you wish to ignore. You can also omit trailing values in a data pattern.Therefore, the previous query is equivalent to this next query, because we ignore the transaction part of the datoms.

[:find ?e
 :where [?e :person/name "Ridley Scott" _]]

Omitting the trailing _, it becomes:

[:find ?e
 :where [?e :person/name "Ridley Scott"]]

Learn more at:

Query parameters

We can pass in query parameters as the 2nd argument. Here we pass in red as the ?color (as per :in ?color). We then find ?e (the entity id) and the ?rgb value of any entity which has a :color/name attribute matching the ?color passed :in.

(d/q '[ :find ?e ?rgb
        :in ?color
        :where [?e :color/name ?color]
      ]
      'red'
;; => [[27 '#ff0000'] [42 '#ff0205']]

We can also pass a list of values as query parameters, using the special [?x ...] form. Here we find the maximum value of a list of integers passed in. See more on aggregation functions below.

(d/q '[ :find max(?x)
        :in [?x ...]
      ]
      [1 2 3 4] ;; query params

;; => [4]

You can learn more at:

Aggregation

Aggregation is possible via functions just like in most other DBs. We will be using the aggregation functions max and min.

In the :find we demand to return the ?color (name) and the max and min values (denoted as ?x) of that color.

The :in clause defines how to use the query parameters. We tell the query to expect a list of tuples [[?color ?x]] Then we pass in the list of tuples: [:red 10] ... [:red 50] and the query result is as expected.

(d/q '[ :find ?color (min ?x) (max ?x)
        :in [[?color ?x]]
      ]
      [[:red 10]  [:red 20] [:red 30] [:blue 20] [:blue 50]] ;; query params

;; => [[:red  [10 30]] [:blue  [20 50]]]