support selectDistinct queries

[dpetran]

Description

When selectDistinct is used in lieu of the select keyword in a query, e.g.

@(fluree/query db1 {:selectDistinct {'?s [:*]} :where [['?s '?p '?o]]})

;; instead of

@(fluree/query db1 {:select {'?s [:*]} :where [['?s '?p '?o]]})

the results of selectDistinct should be unique/distinct. Consider a property, :ex/favNums, that is multi-cardinality. In the following :where clause, we are looking for every subject with a predicate-object pair on :ex/favNums

    {:select '?s
     :where  [['?s :ex/favNums '?o]]}

If one subject has two triples for this property (i.e. two different favNums), then the :select '?s portion of the query would return duplicates of the same subject, because two identical values were bound to '?s given the conditions of the :where clause. In this case, :selectDistinct should return only one instance of each identical result.

Acceptance Criteria

• :selectDistinct is a valid keyword in queries and causes returned results to be unique/distinct from one another

Implementation Details