First proposal for the querycat query language (inspired by SPARQL) for querying categorical data

querycat-lang-proposal.md

Query Example

For each customer, return their name and surname, as well as the list of all products this customer bought, which have a maximum price of 150 EUR.

SELECT {
  ?customer cheapItems _:cheapItems ;
            name       ?name        ;
            surname    ?surname     .

  _:cheapItems item ?item .

  ?item name  ?itemName  ;
        price ?itemPrice .
}
WHERE {
  ?customer -12/14 ?order .
  ?order -42/40 ?item .
  ?item 56 ?itemName  ;
        57 ?itemPrice .
        
  FILTER (?itemPrice <= 150)
}

Syntax Explanation

• WHERE clause does pattern matching on the schema category - generates solutions matching this pattern (using triples subject predicate object .)
  • ?customer is a variable named customer
  • 56 is the ID of a morphism, in this case the morphism from Product to Name. Morphisms prefixed with - are duals, i.e. opposite direction.
    • Note: I arbitrarily assigned IDs to the morphisms from the categorical approach PDF from Pavel Koupil.
  • We can form paths using morphisms: -12/14 means to traverse the dual of 12, and then traverse 14.
  • ; is syntactic sugar for having multiple triples with the same subject, equivalent to simply repeating the subject again in the next triple.
  • FILTER removes some solutions from consideration. Only solutions matching the FILTER are returned.
• SELECT clause describes the returned data - it implicitly defines a schema category, and returned data will be instances of this category.
  • Using an alphanumeric string in the predicate position in SELECT defines a new morphism (and implicitly its dual also).
  • Using bound variables will simply substitute that variable's data.
  • _:cheapItems is syntax for a blank node named cheapItems - this is necessary in order to add new objects to the returned schema, otherwise we could only use bound variables.
    • In this case, because _:cheapItems is first defined in the object position, a new instance will be created for each ?customer instance.
    • If _:cheapItems was first defined in the subject position, there would be a single object shared between all customers.