Class Definitions

Overview

A class is a globally-identified category that nodes belong to, formally similar to a "type" or "label," but with semantic structure attached:

• Identified by an IRI. @foaf:Person is shorthand for <http://xmlns.com/foaf/0.1/Person>. Two systems that use the same IRI are talking about the same class.
• Can sit in a hierarchy via SUBCLASS OF. If @ex:Employee SUBCLASS OF @foaf:Person, querying MATCH (n@foaf:Person) returns Employees. This is subclass inference.
• Can be declared disjoint from others via DISJOINT WITH. @ex:Cat DISJOINT WITH @ex:Dog means no node may carry both labels.
• Can be defined by inference via EQUIVALENT TO. @ex:Director EQUIVALENT TO (@ex:directed SOME @ex:Film) means any individual directed at least one film is classified at query time as @ex:Director.

How it compares to LPG:

Example Graph

Run this before working through the examples:

GQL
CREATE GRAPH myOntology WITH ONTOLOGY
USE myOntology
LOAD PREFIX foaf FROM 'http://xmlns.com/foaf/0.1/'
LOAD PREFIX ex FROM 'http://example.org/'

Creating Classes

Basic Form

Define classes without any axioms:

GQL
CREATE CLASS @ex:Country
CREATE CLASS @ex:City

SUBCLASS OF

Use SUBCLASS OF to declare a subclass relationship: every member of the child class is also a member of the parent class.

Hierarchy
@foaf:Person
   └── @foaf:Agent

GQL
CREATE CLASS @foaf:Person
CREATE CLASS @foaf:Agent SUBCLASS OF @foaf:Person

-- Insert an agent
INSERT (@foaf:Agent {name: 'Alice'})

-- Query for @foaf:Person also returns @foaf:Agent
MATCH (n@foaf:Person) RETURN n.name  // Alice

Hierarchies can chain:

Hierarchy
@ex:Person
   └── @ex:Employee
          └── @ex:Manager

GQL
CREATE CLASS @ex:Person
CREATE CLASS @ex:Employee SUBCLASS OF @ex:Person
CREATE CLASS @ex:Manager SUBCLASS OF @ex:Employee

-- Insert a manager
INSERT (@ex:Manager {name: 'Bob'})

-- Query for @ex:Employee also returns @ex:Manager
MATCH (n@ex:Employee) RETURN n.name  // Bob

-- Query for @ex:Person also returns @ex:Employee and @ex:Manager 
MATCH (n@ex:Person) RETURN n.name    // Bob

A class can have multiple direct superclasses (multi-parent hierarchy). List the parents after SUBCLASS OF, separated by commas. A node of the subclass is automatically inferred to belong to every ancestor class, and a query for any parent returns it.

Two parallel roles combined into one class:

Hierarchy
@ex:Student    @ex:Staff
       \          /
   @ex:TeachingAssistant

GQL
CREATE CLASS @ex:Student
CREATE CLASS @ex:Staff
CREATE CLASS @ex:TeachingAssistant SUBCLASS OF @ex:Student, @ex:Staff

-- Insert a TeachingAssistant
INSERT (@ex:TeachingAssistant {name: 'Amy'})

-- Query for @ex:Student also returns @ex:TeachingAssistant
MATCH (n@ex:Student) RETURN n.name  // Amy

-- Query for @ex:Staff also returns @ex:TeachingAssistant
MATCH (n@ex:Staff) RETURN n.name    // Amy

DISJOINT WITH

Declare two classes mutually exclusive — no node may carry both labels:

GQL
CREATE CLASS @ex:Cat
CREATE CLASS @ex:Dog DISJOINT WITH @ex:Cat

By default, ontology violations are logged as warnings but the operation proceeds (WARNING mode). Switch to STRICT to make violations fail with an error:

GQL
SET ONTOLOGY ENFORCEMENT STRICT

Now the following insert fails as a node cannot be both @ex:Cat and @ex:Dog:

GQL
-- Error: DISJOINT WITH violation
INSERT (@ex:Cat&@ex:Dog {name: 'Mystery'})

DISJOINT WITH accepts a comma-separated list, so a class can be declared disjoint from several others at once:

GQL
CREATE CLASS @ex:Mammal
CREATE CLASS @ex:Bird DISJOINT WITH @ex:Mammal
CREATE CLASS @ex:Fish DISJOINT WITH @ex:Mammal, @ex:Bird

SUBCLASS OF and DISJOINT WITH can be combined:

GQL
CREATE CLASS @ex:Contractor
CREATE CLASS @ex:Intern SUBCLASS OF @ex:Person DISJOINT WITH @ex:Contractor

EQUIVALENT TO

A defined class declares membership by an EQUIVALENT TO axiom instead of an explicit label. Members are inferred at query time from a property restriction. The database classifies entities automatically, with no labeling and no materialization.

To see the contrast, compare a primitive class (you tag the node) with a defined class (the rule tags it for you):

GQL
-- Primitive class: Lana must be explicitly labeled @ex:Director on insert
CREATE CLASS @ex:Director
INSERT (@ex:Director {name: 'Lana'})

MATCH (n@ex:Director) RETURN n.name   // Lana, label was written on the node

GQL
-- Defined class: no @ex:Director label is written anywhere
CREATE CLASS @ex:Film
CREATE OBJECT PROPERTY @ex:directed
CREATE CLASS @ex:Director EQUIVALENT TO (@ex:directed SOME @ex:Film)

-- Any individual who directed at least one film is inferred as a director
-- No @ex:Director label needs to be written
INSERT (lana@ex:Person {name: 'Lana'})-[@ex:directed]->(@ex:Film {name: 'The Matrix'})

MATCH (n@ex:Director) RETURN n.name   // Lana, classified at query time by the rule

If Lana later stops directing films (the edges to @ex:Film are deleted), the next MATCH (n@ex:Director) excludes her — no relabeling required. Useful for derived categories that would otherwise drift out of sync with the data.

EQUIVALENT TO can also combine a class restriction:

GQL
-- A Veteran is a @ex:Person who served in at least one MilitaryBranch
CREATE CLASS @ex:MilitaryBranch
CREATE OBJECT PROPERTY @ex:servedIn
CREATE CLASS @ex:Veteran EQUIVALENT TO @ex:Person AND (@ex:servedIn SOME @ex:MilitaryBranch)

-- An equipment once served is not a veteran
CREATE CLASS @ex:Equipment
INSERT (@ex:Equipment {name: 'Tank-72'})-[@ex:servedIn]->(@ex:MilitaryBranch {name: 'Army 07'})

MATCH (n@ex:Veteran) RETURN n.name   // No data

Supported restriction operators:

GQL
-- A vegetarian restaurant is one whose menu, if it has one, is entirely vegetarian
CREATE CLASS @ex:Dish
CREATE CLASS @ex:VegetarianDish SUBCLASS OF @ex:Dish
CREATE CLASS @ex:Restaurant
CREATE OBJECT PROPERTY @ex:serves DOMAIN @ex:Restaurant RANGE @ex:Dish
CREATE CLASS @ex:VegetarianRestaurant EQUIVALENT TO @ex:Restaurant AND (@ex:serves ONLY @ex:VegetarianDish)

Showing Classes

List all defined classes:

GQL
SHOW CLASSES

Example output:

Filter the output to a single prefix:

GQL
SHOW CLASSES FROM ex

Drill down into one class with SHOW CLASS @prefix:Name — a richer view that adds subclasses, disjoint classes, the properties that name this class as their domain, and a live instance count:

GQL
SHOW CLASS @ex:Person

Returned columns:

Dropping Classes

The RESTRICT mode is applied by default when dropping a class, where the drop fails if any nodes still carry the class label.

GQL
DROP CLASS @ex:Person

-- Equivalent to
DROP CLASS @ex:Person RESTRICT

Use CASCADE to strip the class label from every node that has it:

GQL
DROP CLASS @ex:Person CASCADE

CASCADE removes the label from the affected nodes; the nodes themselves, their other labels, and their properties are left intact. If a node had @ex:Person as its only label, it survives as an unlabeled node.